U.S. patent application number 11/391192 was filed with the patent office on 2007-02-01 for process for preparing p-n junctions having a p-type zno film.
This patent application is currently assigned to The Curators of the University of Missouri. Invention is credited to Yungryel Ryu, Henry W. White, Shen Zhu.
Application Number | 20070022947 11/391192 |
Document ID | / |
Family ID | 23745085 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070022947 |
Kind Code |
A1 |
White; Henry W. ; et
al. |
February 1, 2007 |
Process for preparing p-n junctions having a p-type ZnO film
Abstract
A process for preparing p-n or n-p junctions having a p-type
oxide film is disclosed. In one embodiment, a p-type zinc oxide
film has a net acceptor concentration of at least about 10.sup.15
acceptors/cm.sup.3.
Inventors: |
White; Henry W.; (Columbia,
MO) ; Zhu; Shen; (Huntsville, AL) ; Ryu;
Yungryel; (Columbia, MO) |
Correspondence
Address: |
SENNIGER POWERS
ONE METROPOLITAN SQUARE
16TH FLOOR
ST LOUIS
MO
63102
US
|
Assignee: |
The Curators of the University of
Missouri
Columbia
MO
65211-2015
|
Family ID: |
23745085 |
Appl. No.: |
11/391192 |
Filed: |
March 28, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10615102 |
Jul 8, 2003 |
7033435 |
|
|
11391192 |
Mar 28, 2006 |
|
|
|
10002790 |
Nov 15, 2001 |
6610141 |
|
|
10615102 |
Jul 8, 2003 |
|
|
|
09439529 |
Nov 12, 1999 |
6342313 |
|
|
10002790 |
Nov 15, 2001 |
|
|
|
09364809 |
Jul 30, 1999 |
6410162 |
|
|
09439529 |
Nov 12, 1999 |
|
|
|
09128516 |
Aug 3, 1998 |
6291085 |
|
|
09364809 |
Jul 30, 1999 |
|
|
|
Current U.S.
Class: |
117/89 ; 117/84;
257/E21.462; 257/E29.081; 257/E29.094 |
Current CPC
Class: |
H01L 31/1836 20130101;
H01L 33/285 20130101; H01L 21/2225 20130101; Y02E 10/50 20130101;
H01L 21/02579 20130101; H01L 29/7869 20130101; H01L 29/22 20130101;
H01L 31/02963 20130101; Y10T 428/12681 20150115; C23C 14/086
20130101; H01L 21/02587 20130101; H01L 21/02581 20130101; H01L
21/02403 20130101; H01L 29/267 20130101; H01L 33/28 20130101; C30B
23/02 20130101; H01L 21/0242 20130101; H01L 33/0087 20130101; H01L
21/0256 20130101; H01L 21/02631 20130101; C30B 29/16 20130101; H01L
21/02576 20130101; C23C 14/081 20130101; H01L 21/02395 20130101;
C23C 14/28 20130101; H01L 21/02565 20130101; H01L 21/2255 20130101;
H01S 5/327 20130101; H01L 21/02554 20130101; H01L 21/02661
20130101 |
Class at
Publication: |
117/089 ;
117/084 |
International
Class: |
C30B 23/00 20060101
C30B023/00; C30B 25/00 20060101 C30B025/00; C30B 28/12 20060101
C30B028/12; C30B 28/14 20060101 C30B028/14 |
Goverment Interests
[0001] This invention was made with Government support under
Grant/Project Number DAAH04-94-G-0305 awarded by the Army Research
Office. The Government may have certain rights in the invention.
This application is a divisional application of application Ser.
No. 10/615,102 filed on Jul. 8, 2003 which is a continuation
application of U.S. Pat. No. 6,610,141 issued Aug. 26, 2003 which
is a divisional application of U.S. Pat. No. 6,342,313 issued Jan.
29, 2002 which is a continuation-in-part application of U.S. Pat.
No. 6,410,162 issued on Jun. 25, 2002 which is a
continuation-in-part application of U.S. Pat. No. 6,291,085 issued
on Sep. 18, 2001.
Claims
1-81. (canceled)
82. An oxide film on a substrate, the oxide film having a net
acceptor concentration at least about 10.sup.17
acceptors/cm.sup.3.
83. The oxide film of claim 82 having a resistivity of no greater
than about 1 ohm-cm, and a Hall Mobility of between about 0.1 and
about 50 cm.sup.2/Vs.
84. The film of claim 82 wherein the film includes an oxide
compound selected from an oxide compound of a Group 2 element, an
oxide compound of a Group 12 element, an oxide compound of Group 2
and Group 12 elements, and an oxide compound of Group 12 and Group
16 elements.
85. The film of claim 82 wherein the film is ZnO.
86. The film of claim 82 wherein the p-type dopant comprises an
element selected from the group consisting of Group 1, Group 11,
Group 5, and Group 15 elements.
87. The film of claim 82 wherein the p-type dopant comprises
nitrogen.
88. The film of claim 82 wherein the p-type dopant comprises
arsenic.
89. The film of claim 82 wherein the p-type dopant comprises
phosphorus.
90. The film of claim 82 wherein the p-type dopant comprises
antimony.
91. The film of claim 82 wherein the film is a component of a
device selected from the group consisting of a light emitting
diode, a laser diode, a p-n-p transistor, a n-p-n transistor, a
field-effect transistor, a p-n junction, a photodetector, a
transducer, and a light emitting device.
92. An oxide film on a substrate, the oxide film having a net
acceptor concentration at least about 10.sup.15 acceptors/cm.sup.3,
a resistivity between about 1 ohm-cm and 10.sup.-4 ohm-cm, and a
Hall Mobility of between about 0.1 and about 50 cm.sup.2/Vs.
93. The film of claim 92 wherein the film includes an oxide
compound selected from an oxide compound of a Group 2 element, an
oxide compound of a Group 12 element, an oxide compound of Group 2
and Group 12 elements, and an oxide compound of Group 12 and Group
16 elements.
94. The film of claim 92 wherein the film is ZnO.
95. The film of claim 92 wherein the p-type dopant comprises an
element selected from the group consisting of Group 1, Group 11,
Group 5, and Group 15 elements.
96. The film of claim 92 wherein the p-type dopant comprises
nitrogen.
97. The film of claim 92 wherein the p-type dopant comprises
arsenic.
98. The film of claim 92 wherein the p-type dopant comprises
phosphorus.
99. The film of claim 92 wherein the p-type dopant comprises
antimony.
100. The film of claim 92 wherein the film is a component of a
device selected from the group consisting of a light emitting
diode, a laser diode, a p-n-p transistor, a n-p-n transistor, a
field-effect transistor, a p-n junction, a photodetector, a
transducer, and a light emitting device.
101. An oxide semiconductor film comprising a p-type dopant that is
a layer in a semiconductor device formed on a substrate, the oxide
semiconductor film having a net acceptor concentration at least
about 10.sup.17 acceptors/cm.sup.3.
102. The film of claim 101 having a resistivity of no greater than
about 1 ohm-cm, and a Hall Mobility of between about 0.1 and about
50 cm.sup.2/Vs.
103. The film of claim 101 wherein the film is formed on a
substrate.
104. The film of claim 101 wherein the film is formed on a device
for attaching electrical leads.
105. The film of claim 101 wherein the film includes an oxide
compound selected from an oxide compound of a Group 2 element, an
oxide compound of a Group 12 element, an oxide compound of Group 2
and Group 12 elements, and an oxide compound of Group 12 and Group
16 elements.
106. The film of claim 101 wherein the film is ZnO.
107. The film of claim 101 wherein the p-type dopant comprises an
element selected from the group consisting of Group 1, Group 11,
Group 5, and Group 15 elements.
108. The film of claim 101 wherein the p-type dopant comprises
nitrogen.
109. The film of claim 101 wherein the p-type dopant comprises
arsenic.
110. The film of claim 101 wherein the p-type dopant comprises
phosphorus.
111. The film of claim 101 wherein the p-type dopant comprises
antimony.
112. The film of claim 101 wherein the semiconductor device is
selected from the group consisting of a light emitting diode, a
laser diode, a p-n-p transistor, a n-p-n transistor, a field-effect
transistor, a p-n junction, a photodetector, a transducer, and a
light emitting device.
113. The film of claim 101 wherein the film has a resistivity
between about 1 ohm-cm and about 10.sup.-4 ohm-cm.
114. An oxide semiconductor film comprising a p-type dopant that is
a layer in a semiconductor device formed on a substrate, the oxide
semiconductor film having a net acceptor concentration of at least
about 10.sup.15 acceptors/cm.sup.3, a resistivity of no greater
than about 1 ohm-cm, and a Hall Mobility of between about 0.1 and
about 50 cm.sup.2/Vs.
115. The film of claim 114 wherein the film is formed on a
substrate.
116. The film of claim 114 wherein the film is formed on a device
for attaching electrical leads.
117. The film of claim 114 wherein the film includes an oxide
compound selected from an oxide compound of a Group 2 element, an
oxide compound of a Group 12 element, an oxide compound of Group 2
and Group 12 elements, and an oxide compound of Group 12 and Group
16 elements.
118. The film of claim 114 wherein the film is ZnO.
119. The film of claim 114 wherein the p-type dopant comprises an
element selected from the group consisting of Group 1, Group 11,
Group 5, and Group 15 elements.
120. The film of claim 114 wherein the p-type dopant comprises
nitrogen.
121. The film of claim 114 wherein the p-type dopant comprises
arsenic.
122. The film of claim 114 wherein the p-type dopant comprises
phosphorus.
123. The film of claim 114 wherein the p-type dopant comprises
antimony.
124. The film of claim 114 wherein the semiconductor device is
selected from the group consisting of a light emitting diode, a
laser diode, a p-n-p transistor, a n-p-n transistor, a field-effect
transistor, a p-n junction, a photodetector, a transducer, and a
light emitting device.
125. The film of claim 114 wherein the film has a resistivity
between about 1 ohm-cm and about 10.sup.-4 ohm-cm.
Description
BACKGROUND OF THE INVENTION
[0002] This invention is directed to oxide films, such as zinc
oxide (ZnO) films, for use in electrically excited devices such as
light emitting devices (LEDs), laser diodes (LDs), field effect
transistors (FETs), photodetectors, and transducers. More
particularly, this invention is directed to oxide films containing
a p-type dopant for use in LEDs, LDs, FETs, and photodetectors
wherein both n-type and p-type materials are required, for use as a
substrate material for lattice matching to other materials in such
devices, and for use as a layer for attaching electrical leads.
[0003] For some time there has been interest in producing wide band
gap semiconductors to produce green/blue LEDs, LDs and other
electrical devices. Historically, attempts to produce these devices
have centered around zinc selenide (ZnSe) or gallium nitride (GaN)
based technologies. However, these approaches have not been
entirely satisfactory due to the short lifetime of light emission
that results from defects, and defect migration, in these
devices.
[0004] Recently, because ZnO has a wide direct band gap of 3.3 eV
at room temperature and provides a strong emission source of
ultraviolet light, ZnO thin films on suitable supporting substrates
have been proposed as new materials for light emitting devices and
laser diodes. Undoped, as well as doped ZnO films generally show
n-type conduction. Impurities such as aluminum and gallium in ZnO
films have been studied by Hiramatsu et al. who report activity as
n-type donors (Transparent Conduction Zinc Oxide Thin Films
Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci. Technol. A
16(2), March/April 1998). Although n-type ZnO films have been
available for some time, the growth of p-type ZnO films necessary
to build many electrical devices requiring p-n junctions has to
date been much slower in developing.
[0005] Minegishi et al. (Growth of P-Type ZnO Films by Chemical
Vapor Deposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A
(1997)) recently reported on the growth of nitrogen doped ZnO films
by chemical vapor deposition and on the p-type conduction of ZnO
films at room temperature. Minegishi et al. disclose the growth of
p-type ZnO films on a sapphire substrate by the simultaneous
addition of NH.sub.3 in carrier hydrogen and excess Zn in source
ZnO powder. When a Zn/ZnO ratio of 10 mol % was used, secondary ion
mass spectrometry (SIMS) confirmed the incorporation of nitrogen
into the ZnO film, although the nitrogen concentration was not
precisely confirmed. Although the films prepared by Minegishi et
al. using a Zn/ZnO ratio of 10 mol % appear to incorporate a small
amount of nitrogen into the ZnO film and convert the conduction to
p-type, the resistivity of these films is too high for application
in commercial devices such as LEDs or LDs. Also, Minegishi et al.
report that the carrier density for the holes is
1.5.times.10.sup.16 holes/cm.sup.3. The combined effect of the low
carrier density for holes and the high value for the resistivity
does not permit this material to be used in commercial light
emitting devices or laser diodes.
[0006] Park et al. in U.S. Pat. No. 5,574,296 disclose a method of
producing thin films on substrates by doping IIB-VIA semiconductors
with group VA free radicals for use in electromagnetic radiation
transducers. Specifically, Park et al. describe ZnSe epitaxial thin
films doped with nitrogen or oxygen wherein ZnSe thin layers are
grown on a GaAs substrate by molecular beam epitaxy. The doping of
nitrogen or oxygen is accomplished through the use of a free
radical source which is incorporated into the molecular beam
epitaxy system. Using nitrogen as the p-type dopant, net acceptor
densities up to 4.9.times.10.sup.17 acceptors/cm.sup.3 and
resistivities less than 15 ohm-cm were measured in the ZeSe film.
The combined effect of the low value for the net acceptor density
and the high value for the resistivity does not permit this
material to be used in commercial devices such as LEDs, LDs, and
FETs.
[0007] Although some progress has recently been made in the
fabrication of p-type doped oxide films which can be utilized in
the formation of p-n junctions, a need still exists in the industry
for oxide films which contain higher net acceptor concentrations
and possess lower resistivity values.
SUMMARY OF THE INVENTION
[0008] Among the objects of the present invention, therefore, are
the provision of an oxide film containing a high net acceptor
concentration on a substrate; the provision of a process for
producing oxide films containing p-type dopants; the provision of a
process for producing p-n junctions utilizing an oxide film
containing a p-type dopant; the provision of a process for
producing homoepitaxial and heteroepitaxial p-n junctions utilizing
an oxide film containing a p-type dopant; and the provision of a
process for cleaning a substrate prior to growing a film on the
substrate.
[0009] Briefly, therefore, the present invention is directed to a
ZnO film on a substrate wherein the film contains a p-type dopant.
The film has a net acceptor concentration of at least about
10.sup.15 acceptors/cm.sup.3, a resistivity less than about 1
ohm-cm, and a Hall Mobility of between about 0.1 and about 50
cm.sup.2/Vs.
[0010] The invention is further directed to a process for growing a
p-type ZnO film containing arsenic on a GaAs substrate. The GaAs
substrate is first cleaned to ensure that the film will have a
reduced number of defects and will properly adhere to the
substrate. After cleaning, the temperature of the substrate in the
chamber is adjusted to between about 300.degree. C. and about
450.degree. C. and the excimer pulsed laser is directed onto a
polycrystalline ZnO crystal to grow a film on the substrate. The
temperature of the substrate coated with the film in the deposition
chamber is then increased to between about 450.degree. C. and about
600.degree. C. and the substrate is annealed for a time sufficient
to diffuse arsenic atoms into the film so as to produce a net
acceptor concentration of at least about 10.sup.15
acceptors/cm.sup.3 in the film.
[0011] The invention is further directed to a process for growing a
p-type zinc oxide film on a substrate. The substrate is first
cleaned to ensure that the film will have a reduced number of
defects and will properly adhere to the substrate. After cleaning
the substrate, the temperature of the substrate in the chamber is
adjusted to between about 300.degree. C. and about 450.degree. C.,
and a p-type zinc oxide film is grown on the substrate by directing
an excimer pulsed laser beam onto a pressed ZnO powder pellet
containing a p-type dopant to grow a p-type zinc oxide film
containing a net acceptor concentration of at least about 10.sup.15
acceptors/cm.sup.3.
[0012] The invention is further directed to a process for preparing
a p-n junction having a p-type ZnO film and an n-type film wherein
the net acceptor concentration is at least about 10.sup.15
acceptors/cm.sup.-3. A substrate is loaded into a pulsed laser
deposition chamber and cleaned to ensure that the film will have a
reduced number of defects and will properly adhere to the
substrate. The temperature of the substrate in the deposition
chamber is then raised to between about 300.degree. C. and about
450.degree. C. Subsequently a p-type ZnO film having a net acceptor
concentration of at least about 10.sup.15 acceptors/cm.sup.3 is
grown on the substrate by directing an excimer laser onto a pressed
ZnO powder pellet containing the p-type dopant. Finally an n-type
film is grown on top of the p-type film by directing an excimer
laser beam onto a pressed ZnO pellet containing the n-type
dopant.
[0013] The invention is further directed to a process for preparing
a p-n junction having a p-type ZnO film and an n-type film wherein
the net acceptor concentration is at least about 10.sup.15
acceptors/cm.sup.-3. A substrate is loaded into a pulsed laser
deposition chamber and cleaned to ensure that the film will have a
reduced number of defects and will properly adhere to the
substrate. The temperature of the substrate in the deposition
chamber is then raised to between about 300.degree. C. and about
450.degree. C. Subsequently an n-type film is grown on the
substrate by directing an excimer pulsed laser beam onto a pressed
powder pellet containing an n-type dopant element. Finally, a
p-type ZnO film is grown on the n-type film by directing an excimer
pulsed laser beam onto a pressed ZnO powder pellet containing a
p-type dopant element to a p-type ZnO film having a net acceptor
concentration of at least about 10.sup.15 acceptors/cm.sup.3.
[0014] The invention is further directed to a process for cleaning
a substrate prior to growing a film on the substrate. A substrate
is loaded into a chamber, the temperature of the substrate is
adjusted to between about 400.degree. C. and about 500.degree. C.,
and the chamber is filed with hydrogen to create a pressure between
about 0.5 and about 3 Torr. The distance between a metal shutter in
the chamber and the substrate is adjusted to between about 3 and
about 6 centimeters and an excimer pulsed laser having an intensity
between about 20 and about 70 mJ and a repetition of between about
10 to about 30 Hz is directed onto the shutter for a period of
between about 5 and about 30 minutes to clean the substrate.
[0015] The invention is still further directed to a p-type film on
a substrate wherein the film contains a p-type dopant element which
is the same element as one constituent of the substrate.
[0016] The invention is further directed to a process for preparing
a p-n junction having a p-type ZnO film and an n-type ZnO film on a
p-type doped substrate wherein the net acceptor concentration is at
least about 10.sup.15 acceptors/cm.sup.3. The process comprises
adjusting the temperature of the substrate in a pulsed laser
deposition chamber to between about 300 and about 450.degree. C.
and growing a p-type ZnO film on the substrate by directing an
excimer pulsed laser beam onto a pressed ZnO powder pellet
containing a p-type dopant and growing an n-type film on top of the
p-type film.
[0017] The invention is further directed to a process for growing a
doped ZnO film on a substrate. The process comprises adjusting the
temperature of the substrate in a pulsed laser deposition chamber
to between about 300 and about 450.degree. C. and pre-ablating a
polycrystalline ZnO crystal. Finally, an excimer pulsed laser beam
is directed onto the polycrystalline ZnO crystal to grow a film on
the GaAs substrate while a molecular beam containing a dopant is
simultaneously directed onto the growing ZnO film for a time
sufficient to incorporate at least about 10.sup.15
dopant/cm.sup.3.
[0018] The invention is still further directed to an oxide film on
a substrate. The oxide film contains a p-type dopant and has a net
acceptor concentration of at least about 10.sup.15
acceptors/cm.sup.3, a resistivity no greater than about 1 ohm-cm,
and a Hall Mobility of between about 0.1 and about 50
cm.sup.2/Vs.
[0019] Other objects and features of this invention will be in part
apparent and in part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic diagram of a pulsed laser deposition
system.
[0021] FIG. 2 is a photoluminescence spectra at 20.degree. K of a
ZnO film and an arsenic-doped ZnO film prepared in accordance with
the present invention.
[0022] FIG. 3 is a Secondary Ion Mass Spectroscopy (SIMS) plot of
an arsenic doped ZnO film prepared in accordance with the present
invention.
[0023] FIG. 4 is an Atomic Force Microscopy image of a ZnSe film on
a GaAs substrate wherein the substrate was cleaned using the
cleaning process of the present invention.
[0024] FIG. 5 is an Atomic Force Microscopy image of a ZnSe film on
a GaAs substrate wherein the substrate was cleaned using a thermal
process.
[0025] FIG. 6 is a table showing various electrical properties of
an aluminum doped ZnO n-type film.
[0026] FIG. 7 is a current voltage measurement made on an aluminum
doped ZnO n-type film.
[0027] FIG. 8 is a current voltage measurement made on an arsenic
doped ZnO p-type film.
[0028] FIG. 9 is a current voltage measurement made on a p-n
junction.
[0029] FIG. 10 is a schematic diagram of a p-n junction.
[0030] Corresponding reference characters indicate corresponding
parts throughout the drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] In accordance with the present invention, it has been
discovered that oxide films, such as ZnO films, containing high
levels of p-type dopants can be grown on substrates utilizing a
pulsed laser deposition process alone or in combination with an
annealing step. Surprisingly, the p-type dopant level achieved in
the oxide film is sufficient to allow the p-type film to be used to
form p-n junctions useful in electrical and electroluminescent
devices, for use as a substrate material for lattice matching to
materials in such devices, and for use as a desirable layer for
attaching electrical leads.
[0032] Referring now to FIG. 1, there is shown a schematic diagram
of a pulsed laser deposition system. Such a system is one method
that can be utilized to grow oxide films containing p-type dopants
on suitable substrates in accordance with the present invention.
Other methods of growing oxide films containing p-type dopants on
substrates may include molecular beam epitaxy (MBE), MBE in
conjunction with laser ablation, and chemical vapor deposition
(CVD). Oxide films that may be grown in accordance with the present
invention include oxide compounds of Group 2 (also known as IIA,
which includes elements such as Be, Mg, Ca Sr, Ba, and Ra), oxide
compounds of Group 12 (also known as IIB, which includes elements
such as Zn, Cd and Hg), oxide compounds of Group 2 and Group 12,
and oxide compounds of Group 12 and Group 16 (also known as VIA,
which includes elements such as O, S, Se, Te and Po). Examples of
suitable oxides include BeO, CaO, SrO, BaO, MgO, CdO, ZnO,
Zn.sub.1-xMg.sub.xO, Zn.sub.1-xBa.sub.xO, Zn.sub.l-xCa.sub.xO,
Zn.sub.1-xCd.sub.xO, ZnSe.sub.xO.sub.1-x, ZnS.sub.xO.sub.1-x, and
ZnTe.sub.xO.sub.1-x, with ZnO being most preferred. Suitable p-type
dopants for use in the present invention include Group 1 (also
known as IA, which includes elements such as Li, Na, K, Rb, and
Cs), Group 11 (also known as IB, which includes elements such as
Cu, Ag, and Au), Group 5 (also known as VB, which includes elements
such as V, Nb, and Ta), and Group 15 (also known as VA, which
includes elements such as N, P, Sb, As, and Bi), with arsenic being
preferred.
[0033] Again referring to FIG. 1, there is shown a focusing lens 8
capable of directing an excimer laser beam 10 through laser window
6 into pulsed laser deposition chamber 2. The beam 10 can be
directed onto either metal shutter 4 or target 18 depending upon
the desired processing step. The beam 10 impinges on either metal
shutter 4 or produces an ablation plume of oxide material from the
target 18 and onto substrate 12. During the process of growing
oxide films, gas inlet tube 14 allows gas 16 into the chamber
2.
[0034] Before growth of the oxide film on the substrate, the
substrate should be cleaned in order to remove surface contaminants
such as oxygen and carbon to minimize the number of defects in the
film and to ensure maximum adherence of the film to the substrate.
Conventional substrate cleaning techniques including wet chemical
treatments, thermal cleaning, hydrogen atom plasma treatments, or
any combination thereof can be used to sufficiently clean the
substrate surface. In addition, a pulsed excimer laser, such as a
pulsed argon fluoride excimer laser, can be used to clean the
substrate in situ.
[0035] To clean the substrate using a pulsed excimer laser, the
temperature of the substrate in the pulsed laser deposition chamber
2 is first adjusted to between about 300.degree. C. and about
600.degree. C., more preferably between about 400.degree. C. and
about 500.degree. C., most preferably to about 450.degree. C., and
the chamber 2 is filled with a gas such as hydrogen to create a
pressure of between about 0.5 and about 3 Torr, preferably between
about 1 to about 2 Torr. Referring again to FIG. 1, a metal shutter
4, which may be made from iron, for example, is inserted between
the target 18 and the substrate 12 such that the substrate is
positioned between about 3 and about 6 centimeters, preferably
about 4 centimeters, in front of the metal shutter 4. The focusing
lens 8 is removed from the system, and an excimer laser beam 10,
such as an argon fluoride excimer laser beam having an intensity of
between about 20 and about 70 ml, preferably about 50 mJ and a
repetition rate of about 10 to about 30 Hz, preferably about 20 Hz,
is directed into the chamber 2 and onto the metal shutter 4 for a
period of between about 5 and about 30 minutes, preferably about 10
minutes.
[0036] During this period of illumination of the metal shutter, the
laser beam interacts with the metal shutter and creates excited
hydrogen atoms, photoelectrons, and photons that effectively remove
contaminants from the substrate surface. Using the pulsed excimer
laser, the substrate surface can be effectively cleaned at a much
lower temperature than that required by conventional techniques.
The pulsed excimer laser cleaning process can be effectively
utilized to clean GaAs, GaN, sapphire, and other substrates prior
to the deposition of oxide films such as ZnO or other films such as
ZnSe, GaN. For example, FIGS. 4 and 5 show Atomic Force Microscopy
(AFM) images of the surface morphology of a ZnSe film on GaAs
substrates. In FIG. 4, the GaAs substrate was cleaned prior to the
deposition of the ZnSe film using the cleaning process described
above. The deposited ZnSe film has a thickness of about 0.5
micrometers and has only a root mean surface roughness of about
1.05 nanometers. In FIG. 5, the GaAs substrate was cleaned prior to
the deposition of the ZnSe film by a thermal treatment process at a
substrate temperature of about 570.degree. C. The deposited ZnSe
film has a thickness of about 0.5 micrometers and has a root mean
surface roughness of about 6.65 nanometers. As indicated in FIGS. 4
and 5, the cleaning process of the present invention leaves a much
improved uniform surface for subsequent film growth.
[0037] After the period of illumination is complete, the hydrogen
gas is pumped out of chamber 2, and the temperature of the
substrate is adjusted to between about 200.degree. C. and about
1000.degree. C., preferably between about 250.degree. C. and about
500.degree. C., and most preferably to between about 300.degree. C.
and about 450.degree. C., to grow the oxide film.
[0038] After the substrate has been cleaned and the temperature in
the chamber adjusted, the focusing lens 8 is replaced, the metal
shutter 4 is removed and the target is pre-ablated with a pulsed
excimer laser having an intensity of between about 20 and about 70
mJ, preferably about 50 mJ, and a repetition rate of between about
10 and about 30 Hz, preferably about 20 Hz for a period of about 10
minutes.
[0039] After the pre-ablation is complete, the chamber 2 is filled
with oxygen to a pressure of between about 10.sup.-6 Torr and about
10.sup.-2 Torr, preferably about 35 mTorr. The laser beam 10 is
directed through focusing lens 8 and laser window 6 onto the target
18 to produce an ablation plume of oxide material, such as ZnO for
example, that is adsorbed onto substrate 12. The target 18 is
between about 5 and about 20 centimeters, preferably about 7
centimeters from the substrate 12. Suitable targets for use in the
present invention include polycrystalline oxide containing
compounds such as polycrystalline ZnO, for example, as well as
powder pellets, such as ZnO, containing a dopant. Suitable
substrates for use in the present invention include gallium
arsenide, sapphire, and ZnO. The laser beam 10 can have an
intensity of about 90 mJ and a repetition of about 20 Hz, for
example. The laser beam 10 is directed at the target 18 for a
period of about 0.5 to about 4 hours, preferably about 1 to about 2
hours to grow an oxide film on substrate 12 between about 0.5 and
about 3 micrometers thick, preferably about 1 micrometer thick.
[0040] In a particularly preferred embodiment of the present
invention, the target 18 is polycrystalline ZnO, the substrate 12
is gallium arsenide, and the p-type dopant is arsenic. If the
growth of the ZnO film on the gallium arsenide substrate as
described above occurred at a substrate temperature of at least
about 400.degree. C., no further processing steps are necessary,
and the ZnO layer will contain a net acceptor concentration of at
least about 10.sup.15 acceptors/cm.sup.3, preferably at least about
10.sup.16 acceptors/cm.sup.3, more preferably at least about
10.sup.17 acceptors/cm.sup.3, and more preferably between about
10.sup.18 and about 10.sup.21 acceptors/cm.sup.3 as arsenic atoms
will migrate from the gallium arsenide substrate into the ZnO film
during the film growth at a temperature of at least about
400.degree. C. Additionally, the film will have a resistivity of no
more than about 1 ohm-cm, preferably between about 1 and about
10.sup.-4 ohm-cm, and a Hall mobility of between about 0.1 and
about 50 cm.sup.2/Vs.
[0041] If the growth of the oxide film on the gallium arsenide
substrate occurs below about 400.degree. C., a further processing
step (annealing) is required to diffuse arsenic from the substrate
into the oxide film. This annealing step consists of adjusting the
temperature of the substrate in the chamber 2 to between about
450.degree. C. and about 600.degree. C., preferably to about
500.degree. C., and filling the chamber 2 with a gas such as oxygen
at a pressure between about 0.1 Torr and about 1 atmosphere,
preferably about 1 to about 2 Torr. The gallium arsenide substrate
is annealed for a period of between about 10 and about 60 minutes,
preferably about 20 to about 30 minutes to produce a net acceptor
concentration of at least about 10.sup.15 acceptors/cm.sup.3,
preferably at least about 10.sup.16 acceptors/cm.sup.3, more
preferably at least about 10.sup.17 acceptors/cm.sup.3, and most
preferably between about 10.sup.18 acceptors/cm.sup.3 and about
10.sup.21 acceptors/cm.sup.3 from the substrate 12 into the oxide
film.
[0042] Without being bound to a particular theory, in the one
preferred embodiment when arsenic dopant from a GaAs substrate is
caused to diffuse into a ZnO film, superior results are achieved
due in substantial part to the fact that the p-type dopant
elemental source is the substrate itself. The p-type dopant
elemental source is therefore in intimate contact with the film,
which facilitates diffusion more efficiently and to a greater
degree as compared to processes in which the substrate is not the
dopant source. In this particular embodiment, therefore, having the
dopant source be the substrate facilitates achieving the
improvements in net acceptor concentration, resistivity, and Hall
mobility described herein. Also, the cleaning process as described
herein utilized with the preferred film growing and annealing
processes cleans the GaAs surface extremely well to remove
contaminants such as carbon and oxygen without damaging the crystal
structure. The clean, non-damaged surface allows the ZnO film to
grow with improved crystal alignment and with a reduced number of
defects. This cleaning process therefore further facilitates
diffusion of arsenic more efficiently and to a greater degree,
which results in improvements in structural, optical and electrical
properties.
[0043] In an alternative embodiment, oxide films containing p-type
dopants such as arsenic or n-type dopants such as aluminum can be
grown on substrates such as ZnO, GaAs, GaN and sapphire using a
process wherein an excimer pulsed laser beam is directed onto a
pressed powder pellet comprised of the compound to be grown on the
substrate, such as ZnO, while simultaneously directing a molecular
beam of arsenic or aluminum onto the growing oxide film from either
a thermal evaporation source or an arsenic or aluminum containing
gas. The substrate is held at a temperature of between about
200.degree. C. and about 1000.degree. C., preferably between about
300.degree. C. and about 450.degree. C. and is filled with oxygen
at a pressure of about 10.sup.-6 Torr and about 10.sup.-2 Torr. A
pre-ablation step may be employed on the ZnO target as described
above. The combination of the required molecular beam flux and the
length of time required for the molecular beam containing the
dopant to be directed at the substrate is sufficient to achieve a
net acceptor or donor concentration of at least about
10.sup.15/cm.sup.3, more preferably at least about
10.sup.16/cm.sup.3 still more preferably at least about
10.sup.17/cm.sup.3, and still more preferably between about
10.sup.18/cm.sup.3 and about 10.sup.21/cm.sup.3.
[0044] In a further alternative embodiment, oxide films containing
p-type dopants such as arsenic on a substrate can be prepared using
pressed powder pellets as described above which contain a p-type
dopant as the target in the pulsed laser deposition chamber. This
process does not require migration of the dopant from the substrate
into the film.
[0045] An oxide film is grown on a suitable substrate using the
pulsed laser deposition method described above except that the
target is a pressed oxide powder pellet as described above that
contains a small amount of elemental p-type dopant. The amount of
dopant, such as arsenic, required in the powder pellet to achieve a
net acceptor concentration level of at least about 10.sup.15
acceptors/cm.sup.3, preferably at least about 10.sup.16
acceptors/cm.sup.3, more preferably at least about 10.sup.17
acceptors/cm.sup.3, and still more preferably between about
10.sup.18 acceptors/cm.sup.3 and about 10.sup.21 acceptors/cm.sup.3
is determined by measuring the amount of dopant in the oxide film
and adjusting the dopant level in the powdered pellet until the net
acceptor concentration of at least about 10.sup.15
acceptors/cm.sup.3, preferably at least about 10.sup.16
acceptors/cm.sup.3, still more preferably at least about 10.sup.17
acceptors/cm.sup.3 and most preferably between about 10.sup.18
acceptors/cm.sup.3 and about 10.sup.21 acceptors/cm.sup.3 is
reached. For example, secondary ion mass spectroscopy (SIMS) can be
used to determine the amount of dopant in the oxide film.
Additionally, Hall measurements in combination with electrical
resistivity measurements can be used to determine whether the oxide
film is p-type or n-type, the net concentration of p-type or n-type
carriers in the oxide film, to determine the Hall mobility of the
carriers, and to determine the electrical resistivity of the oxide
film. One skilled in the art will realize that the amount of dopant
required in the powdered pellet may depend on numerous factors
including operating conditions, distances from the target to the
substrate, the size and shape of the chamber, as well as other
variables during growth.
[0046] The concentration of p-type dopant may be varied within the
p-type film by using more than one target and by selecting the
target source during growth that yields the desired acceptor
concentrations in the ZnO film. Such variations may be desirable in
order to prepare surfaces onto which electrical leads may be
attached that have desirable electrical properties.
[0047] Also in accordance with the present invention homoepitaxial
and heteroepitaxial p-n junctions containing p-type doped ZnO films
may be produced on suitable substrates such as gallium arsenide,
sapphire and ZnO. It will be recognized by one skilled in the art
that the terms "homoepitaxial" and "heteroepitaxial" are commonly
used in the art interchangeably with "homostructural" and
"heterostructural," respectively. The term "homostructural" is
generally used when referring to structures wherein the materials
have the same energy band gap and "heterostructural" is generally
used when referring to structures wherein the materials have
different energy band gaps.
[0048] The substrates may be doped with a p-type dopant to provide
electrical contact to a p-n junction formed on the substrate. It
will be recognized by one skilled in the art that an undoped
substrate could also be used to grow a p-n junction on the undoped
substrate. If the substrate is doped with a p-type dopant, such as
zinc for a GaAs substrate for example, the p-type layer is
deposited on the p-type substrate, and finally the n-type layer is
deposited on the p-type layer. Similarly, if the substrate is
n-type doped then the n-type layer is deposited first and then the
p-type layer. Such configurations avoid any p-n junction formation
between the substrate and the first deposited layer. The substrates
are generally doped with a p or n type dopant to create at least
about 10.sup.15/cm.sup.3, more preferably at least about
10.sup.16/cm.sup.3, still more preferably at least about
10.sup.17/cm.sup.3 and most preferably between about 10.sup.18 and
about 10.sup.21/cm.sup.3. It will again be recognized by one
skilled in the art that if an undoped substrate is used, either the
p-type or n-type film can be first grown on the substrate.
[0049] To produce a homoepitaxial p-n junction, a p-type oxide
layer is first grown on the substrate utilizing a pressed oxide
powder pellet containing a p-type dopant such as arsenic as
described above to obtain a net acceptor concentration of at least
about 10.sup.15 acceptors/cm.sup.3, more preferably at least about
10.sup.16 acceptors/cm.sup.3, still more preferably at least about
10.sup.17 acceptors/cm.sup.3, and most preferably between about
10.sup.18 acceptors/cm.sup.3 and about 10.sup.21
acceptors/cm.sup.3. The concentration of p-type dopant may be
varied across the p-type film by using more than one target and by
selecting the target source during growth that yields the desired
acceptor carrier concentration in the oxide film. Such variations
may be desirable in order to prepare surfaces onto which electrical
leads may be attached that have desirable electrical
properties.
[0050] To complete the homoepitaxial p-n junction, an n-type oxide
film is grown on top of the p-type oxide film on top of the
substrate. The n-type oxide film is grown on top of the p-type
oxide film utilizing a pressed oxide powder pellet containing an
n-type dopant such as aluminum, gallium, or indium as described
above to yield an n-type film having a net donor concentration of
at least about 10.sup.15 donors/cm.sup.3, more preferably at least
about 10.sup.16 donors/cm.sup.3, more preferably at least about
10.sup.17 donors/cm.sup.3, and most preferably between about
10.sup.18 donors/cm.sup.3 and about 10.sup.21 donors/cm.sup.3. As
with the p-type film, the concentration level of the n-type
carriers may be varied across the film by employing more than one
target.
[0051] A heteroepitaxial p-n junction can also be produced in
accordance with the present invention. To prepare a heteroepitaxial
p-n junction, a p-type oxide film is grown on a suitable substrate
as described above and a film containing an n-type dopant is grown
on top of the p-type oxide film. In a heteroepitaxial p-n junction
the values of the band gap energies of the p-type film and the
n-type film are different. The n-type film, may be comprised of a
oxide based material for which the value of the band gap energy has
been changed by addition of suitable elements, or the n-type film
may be another material such as zinc selenide or gallium
nitride.
[0052] The use of heteroepitaxial p-n junctions prepared in
accordance with the present invention provides additional materials
for p-n junction and device fabrication so as to achieve an
expanded range of band gap energies, increased optical tuning
ranges, increased device lifetimes, more desirable processing
parameters and conditions, as well as other advantages that will be
recognized by one skilled in the art.
[0053] It will be recognized by one skilled in the art that,
similar to the preparing of oxide films on a substrate, the
preparation of homoepitaxial and heteroepitaxial p-n junctions can
be accomplished using additional techniques in place of pulsed
laser deposition. Other techniques include MBE, MBE with laser
ablation, CVD, and MOCVD. It will also be recognized that devices
having a more complex structure such as n-p-n transistors, p-n-p
transistors, FETs, photodetectors, lattice matching layers, and
layers on which electrical leads may be attached can easily be
fabricated using the above-described techniques and processes.
[0054] In accordance with the present invention, p-type oxide
material may be used as substrate material to reduce or eliminate
problems associated with lattice mismatch. P-type oxide material
that has a sufficiently high net acceptor concentration and low
electrical resistivity can be used for forming electrical contacts
with desirable properties on devices. For example, a template
p-type oxide layer can be synthesized on two-compound semiconductor
substrates such as GaAs. This template would provide a transition
layer for growing epitaxial GaN-based materials with a density of
defects that is lower than would occur in GaN films grown directly
on GaAs.
[0055] The present invention is illustrated by the following
example which is merely for the purpose of illustration and is not
to be regarded as limiting the scope of the invention or manner in
which it may be practiced.
EXAMPLE 1
[0056] In this example a ZnO film was synthesized on a gallium
arsenide substrate and the film/substrate was annealed to diffuse
p-type arsenic dopant from the substrate into the film to produce a
p-type ZnO film on a gallium arsenide substrate.
[0057] A gallium arsenide substrate having the shape of a thin
wafer and being about 1 centimeter by about 1 centimeter by about
0.05 centimeters was loaded into a pulsed laser deposition chamber,
the temperature of the substrate set at 450.degree. C., and the
chamber filled with high purity hydrogen to a pressure of about 2
Torr. An iron shutter was inserted in front of the gallium arsenide
substrate to create a separation distance of 4 centimeters between
the substrate and the shutter. An argon fluoride excimer pulsed
laser beam having an intensity of 50 mJ and a repetition rate of 20
Hz was directed at the metal shutter through a laser window and the
shutter was illuminated for about 20 minutes to clean the
substrate. Subsequently, the hydrogen was pumped out of the
chamber, and the substrate temperature was decreased to about
300.degree. C.
[0058] After the substrate was cleaned, the metal shutter was
removed and a focusing lens was inserted in front of the laser
window to focus the laser beam. The polycrystalline ZnO target was
pre-ablated with the excimer pulsed laser beam which was operating
at an intensity of about 50 mJ and having a repetition of about 20
Hz for a period of about 10 minutes. High purity oxygen gas was
then introduced into the chamber to create a pressure of about 35
mTorr.
[0059] The excimer pulsed laser beam, operating at an intensity of
about 90 mJ and a repetition of about 20 Hz, was then directed at
the polycrystalline ZnO for a period of about 2 hours to grow a ZnO
film having a thickness of about 1.0 micrometers on the
substrate.
[0060] After the film growth, the oxygen gas pressure in the
chamber was adjusted to about 2 Torr, and the temperature of the
substrate is increased to about 500.degree. C. The film/substrate
was annealed for about 30 minutes to diffuse arsenic atoms from the
substrate into the ZnO film. The annealing created an arsenic doped
p-type ZnO film on the gallium arsenide substrate.
[0061] FIG. 2 shows a photoluminescence spectra at 20.degree. K of
the ZnO film (solid line) and the arsenic-doped ZnO film (dots)
prepared in this Example. The pumping excitation is from a pulsed
nitrogen laser with a power density of 128 kW/cm.sup.2. The spectra
shows that for the ZnO film the donor-bound excitonic peaks located
at about 3.36 eV (3698 angstroms) are dominant. However, the
arsenic doped ZnO film of the present example shows that the
acceptor-bound excitonic peak located at about 3.32 eV (3742
angstroms) is the strongest peak. This feature of acceptor-bound
excitonic peaks indicates that the acceptor density is greatly
increased with arsenic doping, and the ZnO film becomes p-type.
[0062] FIG. 3 shows a Secondary Ion Mass Spectroscopy (SIMS) plot
of the arsenic doped ZnO film prepared in this Example. The plot
shows the concentration in atoms/cm.sup.3 of arsenic as a function
of depth from the surface of the arsenic doped ZnO film. This plot
shows that the arsenic concentration is about 10.sup.18
atoms/cm.sup.3 to about 10.sup.21 atoms/cm.sup.3 throughout the
film.
EXAMPLE 2
[0063] In this example a p-n junction, such as the p-n junction
shown in FIG. 10, was synthesized utilizing p-type and n-type ZnO
material on a zinc doped gallium arsenide substrate. The electrical
properties of the p-n junction were measured and electrical data
gathered to demonstrate that the device fabricated shows p-n
junction behavior.
[0064] A zinc doped (0001) gallium arsenide substrate having the
shape of a thin wafer and being about 1 centimeter by about 1
centimeter by about 0.05 centimeters was loaded into a pulsed laser
deposition chamber. To clean the substrate the temperature of the
substrate was set at 450.degree. C., and the chamber filled with
high purity hydrogen gas to create a pressure of about 2 Torr. An
iron shutter was inserted in front of the gallium arsenide
substrate to create a separation distance of about 4 centimeters
between the substrate and the shutter. An argon fluoride excimer
pulsed laser beam having an intensity of about 50 mJ and a
repetition rate of about 20 Hz was directed at the metal shutter
through a laser window (in the absence of a focusing lens) and the
shutter was illuminated for about 20 minutes to clean the
substrate. Subsequently, the hydrogen was pumped out of the
chamber, and the temperature of the substrate in the chamber was
lowered to about 400.degree. C. in preparation for film growth.
[0065] After the substrate was cleaned, a focusing lens was
inserted in front of the laser window to focus the laser beam onto
the polycrystalline ZnO target. The polycrystalline ZnO target was
pre-ablated with the excimer pulsed laser beam which was operating
at an intensity of about 50 mJ and having a repetition of about 20
Hz for a period of about 10 minutes. The metal shutter was in place
between the target and the substrate to protect the substrate from
contamination during pre-ablation. After the pre-ablation was
completed, the metal shutter was removed and high purity oxygen was
introduced as an ambient gas to create a pressure of about 40 mTorr
during p-type film growth.
[0066] The excimer pulsed laser beam, operating at an intensity of
about 90 mJ and a repetition of about 20 Hz, was then directed at
the polycrystalline ZnO for a period of about 2 hours to grow a ZnO
film having a thickness of about 1.5 micrometers on the substrate.
The separation distance between the substrate and the target was
about 7 cm. After the laser beam was shut off, the substrate
temperature was adjusted to about 450.degree. C. and held for 20
minutes in an ambient gas pressure of about 40 mTorr. Finally, the
temperature was decreased to about 350.degree. C., and the n-type
layer growth steps initiated.
[0067] For growth of the n-type layer on top of the p-type layer,
the target was replaced with an alloy of ZnO and aluminum oxide
(Al.sub.2O.sub.3) wherein the Al.sub.2O.sub.3 was about 2% by
atomic weight. The metal shutter was placed between the target and
the substrate, the temperature of the substrate adjusted to about
350.degree. C., and the oxygen pressure adjusted to 40 mTorr. The
laser was operating at an intensity of 50 mJ and a repetition rate
of 20 Hz. The target was pre-ablated for a period of 20
minutes.
[0068] After pre-ablation, the metal shutter was removed and the
laser beam adjusted to an intensity of 90 mJ with a repetition rate
of 20 Hz and was focused on the alloy target for a period of about
2 hours to grow a film having a thickness of about 1.5 micrometers.
The distance between the target and the substrate was about 7 cm.
After growth, the laser beam was shut off and the substrate having
the p-type and n-type layer cooled to room temperature.
[0069] FIG. 6 shows electrical properties of an n-type layer of an
aluminum doped ZnO film grown on an undoped GaAs substrate
utilizing the same process described for growth of the n-type ZnO
layer in Example 2. The data is presented over a range of magnetic
fields from 1001 Gauss to 5004 Gauss as shown in the Field column
at a temperature of about 290 Kelvin. The Hall Coefficient values
are negative values throughout the entire Gauss range indicating
that the net carrier concentration is negative and the material is
n-type. The resistivity values are low and indicate that the n-type
film has electrical conductivity properties sufficient for use in
the fabrication of electrical devices. The carrier density values
are negative indicating the film is n-type. Also, these values are
above 10.sup.18 cm.sup.-3 which indicate that the carrier
concentration is sufficient for use in electrical devices. Finally,
the Mobility values are all negative and near the value of 1
cm.sup.2/volt-sec and indicate the film is n-type and has carrier
mobility properties sufficient for fabrication of electrical
devices.
[0070] FIG. 7 shows a current-voltage measurement made on the
aluminum doped ZnO film. FIG. 7 shows that the electrical current
versus applied voltage approximates a straight line from about 8
volts negative to about 8 volts positive (centered about zero)
which demonstrates that the electrical behavior is that of an ohmic
material and not a p-n junction device which would display
rectifying behavior.
[0071] FIG. 8 shows a current-voltage measurement made on an
arsenic doped ZnO film grown on zinc doped GaAs substrate using the
same process as described for growth of the p-type ZnO layer in
Example 1. FIG. 8 shows that the electrical current versus applied
voltage approximates a straight line from about 0.6 volts negative
to about 0.6 volts positive (centered about zero) which
demonstrates that the electrical behavior is that of an ohmic
material and not a p-n junction device. It will be recognized by
one skilled in the art that for p-type material in contact with
p-type material it is sufficient to measure current versus applied
voltage in a more restricted range to demonstrate ohmic
behavior.
[0072] FIG. 9 shows a current-voltage measurement on the entire p-n
junction of Example 2. the fact that the electrical current versus
applied voltage rises above a straight line for applied voltages
greater than 1 volt positive for this device and the fact that the
electrical current versus applied voltage approximates a straight
line for applied voltages to about 2 volts negative demonstrates
that the electrical behavior of the device does not display the
behavior of an ohmic material and does display the electrical
characteristics of a rectifying device and a p-n junction.
[0073] In view of the above, it will be seen that the several
objects of the invention are achieved.
[0074] As various changes could be made in the above-described
process for preparing p-type ZnO films without departing from the
scope of the invention, it is intended that all matter contained in
the above description be interpreted as illustrative and not in a
limiting sense.
* * * * *