Process for producing p-type doped layers, in particular, in II-VI semiconductors

Disclosed is a process for producing p-type doped layers, in particular, in II-VI semiconductors, in which the p-type doped layer is produced in a CVD-step by means of plasma activation of nitrogenated gases.

TECHNICAL FIELD 
The present invention relates to a process for producing p-type doped 
layers, in particular, in II-VI semiconductors. 
In a number of fields of application, it would be advantageous to be able 
to use semiconductor diodes that emit blue light. 
By way of illustration, the use of semiconductor diodes that emit in the 
"blue range" would raise the storage density of the CD plates 
approximately by the factor 4. Furthermore, semiconductor diodes that emit 
blue light may find application in writing on writable/deletable optical 
memories. 
STATE OF THE ART 
By way of example, II-VI semiconductors ZnSe and ZnS having a band gap of 
2.67 eV, respectively 3.66 eV at room temperature as well as ternary 
semiconductors such as e.g., ZnSSe are suited for producing light diodes 
that emit blue light. Semiconductors of this type require p-type doped 
layers. 
Moreover, GaAs based optoelectronic circuits (OEIC) can be improved, i.a., 
with p-type doped ZnSe layers, by way of illustration, serving as a 
"current blocking layer" for lasers or as waveguides. The first optical 
circuits "SEED" (self electro optic effect devices) hitherto presented as 
demonstration models also require p-type doping. 
In p-type doping, in particular, of ZnSe layers, but also other 
zinc-containing layers, difficulties crop up in industrial realization: 
P-type doping of ZnSe can take place by putting atoms of the group I such 
as Li or Na in a Zn position or by putting atoms of the group V, such as 
N, P or As in a Se position. 
The hitherto achieved results are still receiving negative review in some 
surveys and critical articles: 
With regard to this, reference is made to "Electrical properties of twinned 
ZnSe: p-type conductivity and chaos" by G. F. Neumark in Material Science 
Forum Vol. 38-41 (1989) 513-518, "Achievement of low resistivity p-type 
ZnSe and the role of twinning" by G. F. Neumark in J. Appl. Phys. 65 (12) 
(1989) 4859, or "Conductivity control of ZnSe-grown by MOVPE and its 
application for blue electro-luminescence" by H. Kukimoto in J. of Crystal 
Growth 101 (1990) 953. 
Furthermore, a number of attempts of doping with lithium have been 
reported, such as, e.g., by A. Yahata, H. Mitsuhashi, K. Hirahava in 
"Confirmation of p-type conduction in Li-doped ZnSe layers grown on 
GaAs-substrates" in the Jap. J. of Appl. Phys. 29 (1) (1990) L 4 or by H. 
Cheng, J. M. De Puydt, J. E. Potts, T. L. Smith "Growth of p-type ZnSe: Li 
by molecular beam epitaxy" in Appl. Phys. Lett. 52 (2) (1988) 147. 
Optical measurements also show acceptor levels; on the other hand, high 
free hole concentrations larger than 8.times.10.sup.16 cm.sup.-3 and low 
resistances were not found. The diffusion coefficient of lithium in ZnSe 
is very great, with electromigration being observed so that an application 
in components does not look promising. Concerning this reference, is made 
to "Electromigration in p-type ZnSe:Li" by M. A. Haase, J. M. De Puydt, H. 
Cheng, J. E. Potts in Appl. Phys. Lett. 58 (1991) 1173. 
Although sodium has an activation energy of 124 meV in ZnSe, no 
conductivity that is usable for components has been observed, i.e., 
Na-doped ZnSe remains highly resistive, as was reported by T. Yodo, K. 
Veda, K. Morio, K. Yamashita, S. Tanaka in "Photoluminescence study of Li- 
and Na-implanted ZnSe epitaxial layers grown by atmospheric pressure 
metalorganic vapor-phase epitaxy", J. Appl. Phys. 68 (7) (1990) 3212 or by 
W. Stutius in "Growth and doping of ZnSe and ZnS.sub.x Se.sub.1-x by 
organo-metallic chemical vapor deposition", J. of Crystal Growth 59 (1982) 
1. 
Likewise doping ZnSE with phosphor and arsenic usually yields 
high-resistive layers by generating deep traps. Concerning this, reference 
is made to H. Kukimoto's "Conductivity control of ZnSe-grown by MOVPE and 
its application for blue electroluminescence", J. of Crystal Growth 101 
(1990) 953 or to W. Stutius', "Growth and doping of ZnSe and ZnS.sub.x 
Se.sub.1-x by organo-metallic chemical vapor deposition", J. of Crystal 
Growth 59 (1982) 1. 
On the other hand, doping ZnSe with nitrogen seems useful: 
The layer structure of the blue-green laser diodes first published in 
September 1991 by M. A. Haase, J. Quium, J. M. De Puydt, H. Cheng in Appl. 
Phys. Lett. 59 (11) (1991). 1272 also has a nitrogen doping. The 
realization of this laser structure, which emits light at 490 nm, was not 
possible prior to the introduction of nitrogen doping with plasma 
activation in the MBE process. 
In the aforementioned paper, N.sub.A -N.sub.D dopings between 3*10.sup.17 
cm.sup.-3 and 1*10.sup.18 cm-.sup.-3 and specific resistances of 0.75 ohm 
cm were achieved. In addition, reference is made to "p-type ZnSe by 
nitrogen atom beam doping during molecular beam epitaxial growth", by R. 
M. Park, M. B. Troffer, C. M. Rouleaue in Appl. Phys. Lett. 57 (20) (1990) 
2127. 
Very early attempts of nitrogen doping were conducted with MOVPE processes 
(metal organic vapor phase epitaxy); for this, reference is made to 
"Nitrogen as shallow acceptor by organometallic chemical vapor deposition" 
by W. Stutius in Appl. Phys. Lett. 40 (3) (1982) 246-248 or 
"Nitrogen-doped p-type ZnSe films grown by MOVPE", by A. Ohki, M Shibata, 
K. Ando, A. Katsui in J. Crystal Growth 93 (1988) 692. 
Lower resistances than 10.sup.2 ohm cm have, however, not been achieved. 
The reason for this is self-compensation due to deep traps. Theoretical 
calculations show that in a ZnSSe/ZnSe multilayer structure the activation 
of nitrogen can be improved by the factor 4-5 (S. Y. Ren, J. D. Dow, S. 
KLemm "strain-assisted p-type doping of II-VI semiconductors", J. Appl. 
Phys. 66 (5) (1989) 2065 or I. Suemune "Doping in a superlattice 
structure: improved hole activation in wide-gap II-VI materials", J. Appl. 
Phys. 67 (5) (1990) 2364). First improvements were experimentally proven 
in MOVPE structures as I. Suemune, H. Masato, K. Nakanishi, Y. Kuroda, M. 
Yamanishi reported in "Doping of nitrogen in ZnSe films: improved 
structures grown on GaAs by MOVPE" in J. Crystal Growth 107 (1991) 679. 
Although, as was explained in the preceding, various different processes 
for p-type doping, in particular, of II-VI semiconductors have been 
investigated, none of the processes has proven satisfactory in practice. 
DESCRIPTION OF THE INVENTION 
The object of the present invention is to provide a process for producing 
p-type doped layers, in particular, in II-VI semiconductors permitting 
rapid and sure and, in particular, reproduceable production of p-type 
doped layers. 
A solution to this object according to the present invention is set forth 
in claim 1. Further improvements of the present invention are the 
subject-matter of the subclaims. 
An element of the present invention is that the p-type doped layer is 
produced in a CVD-step (chemical vapor deposition) by means of plasma 
activation of nitrogenated gases Preferably, the CVD process is conducted 
in a nitrogen carrier gas, with either a plasma in the nitrogen being 
spontaneously ignited or other nitrogenated compounds are additionally 
injected which are then activated by a plasma (claim 2). 
The doping mechanism functions as follows: nitrogenated gases are 
decomposed by plasma activation and excited nitrogen radicals are 
generated. These nitrogen radicals are inserted into the surface of the 
growing semiconductor and result in doping with suitably high activation. 
In particular, with the invented process, II/VI semiconductors, such as 
ZnSe, ZnS or ternary zinc containing semiconducting compounds, can be 
doped. 
It is preferred if N.sub.2, N.sub.2 H.sub.4, NH.sub.3 and other 
nitrogenated compounds are employed as the starting materials (claim 5). 
The invented fundamental idea is universally applicable: the plasma doping 
can occur in MO (metal organic), MBE (molecular beam epitaxy) or MOCBE 
(metal organic chemical beam epitaxy) processes. Furthermore, the level of 
plasma doping can be varied by changing the temperature and/or the VI/II 
ratio as well as the plasma energy in order to influence the level of the 
doping.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS 
In the following figures, the same or the corresponding parts have the same 
numbering obviating renewed presentation and only deviations of the 
preferred embodiments from the first preferred embodiment shown in these 
figures are explained: 
In order to let ZnSe (100) grow on GaAs-substrate, a horizontal 
low-pressure MOVPE reaction equipment (1) having a DC-plasma source (3) 
(direct current) was employed. The misorientation of the GaAs-substrate 
(2) lies at 2 per cent compared to the next &lt;110&gt; direction. The cathode 
(4) is composed of tantalum wire and the anode (5) is designed as a steel 
cylinder having a diameter of approximately 1 cm and a length of 1 cm. The 
anode (5) surrounds the tantalum wire. The distance between the receiver 
(6) on which the substrate is applied and the. plasma source is 10 cm. 
The vapor pressure of the DASe ((H.sub.2 C=CH-CH.sub.2).sub.2 Se) was 
measured. It is represented by the following equation: 
log(p)=9.6556--2636/T, with T standing for the temperature in Kelvin and p 
the pressure in hPa. This means that if the temperature is 18.degree. C. 
the material has a vapor pressure of 4 hPa, which seems suited for the 
MOVPE process. The growth temperature was varied within a range of 
320.degree. to 600.degree. C., with the overall pressure varying from 9.5 
to 150 hPa. In this case, the VI/II ratio changes from 0.5 to 7.7 and the 
plasma output from 0 to 7W. The same was done for DEZn (diethyl zinc). 
The carrier gas (7) may be composed of, e.g., H.sub.2 or N.sub.2 or 
nitrogen hydride compounds. The reference number 8 stands for the gas 
outlet. The reference numbers 9, 10 and 11 stand for the positions where 
the starting materials, such as selen, N.sub.2 for doping, DEZ or other 
materials can be introduced or have been introduced. 
FIG. 2 shows the growth rates of various starting materials for the growth 
with and without plasma. The growth rates are given in .mu.m/h, the 
inverse growth temperature in 1000/Kelvin. 
Especially preferred in this example of an embodiment are starting material 
combinations for the growth of layers while employing plasma. This becomes 
clear due to the relative same-size growth rate for quite a large 
temperature range. Pointed out are output material combinations such as 
DESe (diethyl selenide) with DEZn, (diisopropyl selenide) (DIPSe) with 
DEZn and/or DEZn with diallyl selenide. 
FIG. 3 shows SIMS measurements for a sample having five different layers 
which were grown at different growth parameters. The number of secondary 
atoms are given in cm.sup.-3 and the depth in .mu.m. Secondary ion mass 
spectra were made of Se, Zn, As, N, Ga and C. Distinctly discernable are 
the different atom concentrations in the different layers. Apparent is the 
advantage of a plasma-supported doping of nitrogen in the layers from a 
relatively small concentration of carbon and gallium as well as a higher 
concentration of nitrogen in relation to other materials. 
In the preceding, the invention is described using preferred embodiments 
without any intention of limiting the scope or spirit of the overall 
inventive concept.