Optically writing erasable conductive patterns at a bandgap-engineered heterojunction

A heterojunction is formed between a pair of layers of different semiconductive materials whose work function difference produces a large band offset at the heterojunction. Donor or acceptor atoms are included in one regions that when photoexcited produce free charge carriers but leave behind charged centers that keep the photoexcited carriers localized. The large barrier at the heterojunction limits recombination of the free charge carriers and the charged centers and persistent photoconductivity results. This effect is used to form light operated switches. An illustrative example uses a layer of high purity gallium arsenide forming a heterojunction with a gallium-doped layer of zinc selenide.

FIELD OF THE INVENTION 
This invention relates to optoelectronic apparatus and more particularly to 
such apparatus of the kind that utilizes a doped compound-semiconductive 
element that is provided with a heterojunction by means of which there can 
be created a photo-induced metastable state in which the photogenerated 
carriers are mobile for a relatively long time while the dopant ions 
remain fixed. 
BACKGROUND OF THE INVENTION 
Currently there has been growing interest in optoelectronic devices that 
make use of persistent photoconductivity that can be created in doped 
semiconductive compounds that contain DX centers. DX centers are formed in 
a semiconductor when a donor impurity does not form a shallow hydrogen 
state, but rather a deep, tightly bound state, owing to a large lattice 
relaxation around the donor ion. In the ground state, the DX center is 
negatively charged. Optical excitation converts it into a normal shallow 
donor, with the release, at high enough temperatures or densities, of two 
electrons into the conduction band. The excited state is metastable, 
because the structural relaxation required to return to the DX ground 
state gives rise to a capture barrier E.sub.CAP. At sufficiently low 
temperatures, this results in persistent photoconductivity: the electron 
concentration remains high indefinitely, even after the exciting light 
source has been removed. Moreover, because of Coulomb forces provided by 
the ionized donor centers, these free electrons remain confined near the 
donor centers. An application Ser. No. 08/225,047 filed Apr. 8, 1994, 
assigned to the same assignee as the instant application describes a 
variety of optoelectronic apparatus that make use of persistent 
photoconductivity or persistent photorefractive effects that are 
associated with DX centers. Unfortunately, however, the ambient 
temperature that has to be maintained to prevent recapture of the 
photocarriers in materials presently known to exhibit DX centers is 
relatively low and room temperature operation is not feasible. This 
restricts the usefulness of such devices. 
SUMMARY OF THE INVENTION 
An object of the present invention is semiconductive storage apparatus 
that, when irradiated by a pattern of light, stores the pattern as a 
persistent pattern of increased conductivity without the need for the low 
temperatures needed by the embodiments described in the aforementioned 
patent application. 
In particular, the present invention uses as the storage medium a 
semiconductive element that includes a special heterojunction between two 
regions of different semiconductive materials of which one is 
appropriately doped with a donor that can introduce DX centers. The 
photocarriers reside at the heterojunction and are spatially removed from 
the original sites of DX centers. If the heterojunction is designed to 
provide a large band offset, the latter serves as a high barrier to 
recombination, since the photocarriers must tunnel through the barrier or 
be thermally excited over it in order to return to the original DX sites. 
If the barrier is high enough (of order 1eV), PPC is feasible which 
persists at room temperature for at least about one year. 
As a specific example, the semiconductive element comprises a region of 
insulating GaAs contiguous with a region of Ga-doped ZnSe to form a 
heterojunction. The gallium atoms form DX centers in the zinc selenide 
and, when ionized by photoexcitation, release free electrons. On the 
gallium arsenide side far from the heterojunction, the Fermi level is in 
the middle of the band gap because it is undoped. On the gallium-doped 
zinc selenide side, also far from the interface, the Fermi level is pinned 
at the level of the deep DX center, when the element is in the dark. This 
is because in general there is some compensation that depletes a fraction 
of the DX centers. 
On photo-excitation, the gallium ions which are negatively charged undergo 
a lattice relaxation and release two electrons to the conduction band. 
These photo-generated electrons will reside at the lowest energy level 
available, which is at the lowest level of the two dimensional electron 
gas (2DEG), if it is arranged to lie lower than the bottom of the zinc 
selenide conduction band after illumination, but higher than the DX level 
in the dark. However, since the photocarriers have left behind a 
positively charged gallium ion in the zinc selenide, they will be 
attracted to the region of photo-excitation by Coulomb interactions and 
thus be spatially confined to those regions of the 2DEG that lie directly 
below the illuminated regions of the zinc selenide layer. 
As an illustrative example, a normally insulating storage medium of the 
kind described is provided with a plurality of input terminals and a 
plurality of output terminals and light is used to create a conductive 
path through the medium between a selected input terminal and a selected 
output terminal. This conductive path persists even after the light is 
extinguished. Moreover, this conductive path is readily erasable for 
example by thermal annealing, when desired, after which a new conductive 
path can be formed connecting a different pair of terminals. 
The invention will be better understood from the following more detailed 
description taken in conjunction with the accompanying drawing.

DETAILED DESCRIPTION OF THE INVENTION 
With reference now to the drawing, FIG. 1 shows the total energy of the 
dopant as a function of the position of the dopant ion. Curve 11 
represents the conduction band or the metastable state of the donor atom 
responsible for the bistable characteristic important to the invention; 
curve 12 shows the stable DX state of the donor atom that has captured an 
electron and become negatively charged. 
Persistent photoconductivity occurs when electrons trapped in the DX states 
are ionized by photons of energy equal to or greater than E.sub.opt shown 
in the drawing, and the DX states become ionized shallow donors, which 
produce a free carrier density, dependent on exposure, up to the original 
doping density. Recapture of the electrons, which requires thermal 
excitation over the capture barrier, E.sub.CAP, does not occur when the 
temperature is too low for the requisite thermal excitation. In the usual 
case of DX centers in a bulk semiconductor, the capture barrier 
(E.sub.CAP) is small (about 0.3eV), and in order to keep the carrier 
concentration high after the exciting light source has been removed, the 
ambient temperature must be kept low, typically below 100K with materials 
presently known to exhibit DX centers. 
In order to achieve room-temperature (300K) operation in the bulk, the 
intrinsic capture barrier E.sub.CAP of the bulk material must be large, 
i.e. about 1 eV. Alternatively, the recombination time can be made very 
long by spatially removing the photocarriers from the DX center so that 
they must tunnel back through a barrier in order to recombine, as in 
accordance with the present invention. Such a barrier is provided by the 
heterojunction included in the semiconductive device shown in FIG. 2. 
The semiconductive device comprises a monolithic structure 20 that includes 
a substrate 21 of high purity (undoped) insulating gallium arsenide on 
which has been grown epitaxially, typically by molecular beam epitaxy 
(MBE), a layer 22 of gallium-doped zinc selenide. Typically the gallium 
concentration in the zinc selenide layer is about 10.sup.16 -10.sup.18 
atoms/cm.sup.-3. The gallium serves to form DX centers in the zinc 
selenide according to 2Ga.sub.H .fwdarw.Ga.sup.+ +DX.sup.-, where Ga.sub.H 
denotes the hydrogenic, shallow donor state, Ga.sup.+ a positively 
charged donor ion, and DX.sup.- the ground state of the DX center, which 
is negatively charged. Upon photoexcitation, the gallium ions undergo a 
lattice relaxation with the release of two electrons for each DX center 
into the conduction band: 
EQU DX.sup.- +h.omega..fwdarw.Ga.sup.+ +2e.sup.-, 
where h.omega. is the energy of the exciting photon. Far into the zinc 
selenide region, the Fermi level is now pinned at the level of the shallow 
hydrogenic donor (see FIG. 3b). But if the thickness of the ZnSe Layer is 
matched to the depletion width, then all the photogenerated electrons 24 
reside at the 2DEG (the two-dimensional gas), since they tend to occupy 
the lowest energy available to them. However, since they have left behind 
in the regions of photoexcitation of layer 22 a positively charged gallium 
ion, they will be attracted to the underlying regions of the layer 21 by 
Coulomb interaction. This is illustrated in FIG. 2 which shows a cross 
section of the heterojunction region 23 under non-uniform illumination 
indicated by the pattern of light rays 25 shown. 
In FIG. 3, there are shown the band diagrams along with the Fermi level in 
the regions of interest of the semiconductive element both before (3A) and 
after (3B) illumination. 
In FIG. 4, there is shown an optically-controlled switch that makes use of 
the PPC in a semiconductive element of the kind described. It includes a 
semiconductive element 30 of the kind described which is provided at one 
edge with a plurality of input terminals 31 and at an opposite edge with a 
plurality of output terminals 32. An appropriate source of illumination is 
made to form illuminated paths selectively between one or more input 
terminals and one or more output terminals to form desired conductive 
paths therebetween. These conductive path will persist even after the 
illumination has been discontinued until the conductive paths are erased 
in any of the ways available for such erasure, such as the thermal 
excitation of the photoelectrons out of the 2DEG back to the empty sites 
of the gallium ions. 
It is the spatial separation of the excited photoelectrons from the gallium 
ions from which they were excited that boosts the annealing temperature of 
the persistent photoconductivity. The photoconductivity persists even at 
high temperature because the recombination of the photogenerated electrons 
is now limited to the electrons either tunneling through the barrier or 
being thermally excited out of the 2DEG back to the empty sites of the 
gallium ions in the layer 22. 
PPC is also found in multiple quantum wells (MQW) in which after 
photoexcitation the electrons reside in one semiconductor well and the 
holes reside in the other. However, in such a case both electrons and 
holes are mobile, and it is not possible to write a persistent pattern of 
spatially varying conductance. The latter is possible in the structure 
proposed here since the DX ions are localized. 
The choice of the materials forming the heterojunction is important to 
ensure that such recombination remain small until it is desired to erase 
the pattern stored. Ideally, in the dark the deep DX level must be well 
below the lowest 2DEG level and after illumination the lowest 2DEG level 
must in turn be below the conduction band of the doped semiconductor to 
capture the photoelectrons. It is also usually important that the two 
compositions forming the heterojunction be relatively well lattice 
matched. In some instances it may be advantageous to use a ternary 
compound as one composition at the heterojunction to facilitate lattice 
matching. 
These two conditions require that the binding energy of the deep DX center 
must be sufficiently large. In addition, a large band offset, which is a 
function of the work functions of the two materials forming the 
heterojunction, is required in order to form a large barrier at the 
heterojunction and thus to boost the annealing temperature of the bistable 
state effect responsible for the persistent photoconductivity. In order to 
achieve an annealing temperature above normal room temperature, a barrier 
of about 1eV is needed. 
One technique for achieving this amount of offset include the use of 
wide-band gap semiconductors, as in the example described, involving a 
heterojunction between a III-V semiconductor, such as gallium arsenide, 
and a wider band gap II-VI semiconductor, such as zinc selenide, whose 
band gaps are illustrated in FIG. 3. Barriers of order 0.8 eV have been 
achieved in GaAs-ZnSe heterojunctions. See, for example, R. Nicolini et 
al, Phys. Rev. Lett., vol. 72, page 294, 1994. 
Other combinations that can provide offsets of the size important for near 
room-temperature operations include InAs/ZnSe, InAs/AlGaAs, Ge/AlGaAs, 
Ge/CdZnTe, and InP/CdS. In practice, the band offset may be varied 
considerably by minor departures from stoichiometry in either region in 
the immediate neighborhood of the heterojunction. For example, in the 
embodiment shown in FIG. 2, it is advantageous to make the portion of 
layer 22 selenium-rich to increase the band offset at the heterojunction. 
It should also be possible to utilize the phenomenon of AX centers in which 
holes are made locally mobile by ionization of acceptor centers in an 
analogous manner to the electrons by donor centers. One example of a 
material which forms AX centers is Zinc-magnesium sulfide -selenide doped 
with nitrogren, which has a band gap of about 3 eV. It will be convenient 
to use the expression "significant impurities" to denote generically 
materials that act either as donors or acceptors for contributing the DX 
and AX centers that give rise to the electrons or holes that serve as the 
free charge carriers responsible for the persistent photoconductivity 
characteristic of the invention. It will also be convenient to describe 
such centers generically as bistable centers. 
It is to be understood that the specific embodiment described is merely 
illustrative of possible applications of the invention. Various other 
embodiments can be devised by a worker in the art without departing from 
the spirit and scope of the invention.