Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2000-01-10
2002-04-16
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C257S078000
Reexamination Certificate
active
06372536
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the semiconductor technology field. More specifically, the present invention relates to a II-VI semiconductor component having at least one junction between a layer containing Se and a layer containing BeTe, and to a process for producing the junction. It relates, in particular, to a low-loss contact with a p-conducting mixed crystal based on II-VI semiconductor materials such as, for example, ZnSe.
A plurality of processes are used for the production of electrical contacts on II-VI semiconductor components. In particular, considerable difficulties are encountered in this regard for such components made of mixed crystals based on ZnSe.
It has been found that, in the production of contacts on p-conducting ZnSe, it is inappropriate to use a simple metal contact. Due to the very high valence band affinity of ZnSe a simple metal contact will always lead to the formation of a high Schottky barrier at the metal/p-type semiconductor junction. Positive charge carriers can tunnel through such a high Schottky barrier only with difficulty.
The tunneling effect can be amplified by increasing the doping of the p-type ZnSe using very low growth temperatures, and thereby narrowing the potential barrier (cf. J. Qiu et al., Journal of Crystal Growth 12 (1993), p. 279 et seq.). Other attempts to increase the conductivity through the p-type contact on ZnSe make use of a highly conductive HgSe layer which is fitted between the metal contact and the p-type ZnSe (cf. Y. Lansari et al., Applied Physics Letters 61 (1992), pages 2554 et seq.).
Increased edge doping can also take place using a near-surface p-type ZnTe layer, which also has lower valence band affinity (corresponds to higher valence band energy) than ZnSe. A flatter Schottky barrier is therefore formed. Holes can overcome the flatter Schottky barrier more easily when the acceptor concentration is high.
Lowering the valence band affinity at the surface in order to improve the ohmic contact properties runs up against the problem that, owing to the different valence band affinity of the component cover layer (for example based on ZnSe), and of the superficial contact layer (consisting, for example, of ZnTe or BeTe), a barrier is created for holes in the semiconductor body which makes transport through the semiconductor contact structure more difficult.
It is known that the valence band discontinuity of isovalent interfaces, for example ZnSe/ZnTe or GaAs/AlAs, can only be adjusted to a small extent (cf. R. G. Dandrea, C. B. Duke, Journal of Vacuum Science and Technology B 10(4) (1992), page 1744). There is accordingly as yet no known method by which the band discontinuity at a junction such as, for example, from ZnTe to ZnSe or from BeTe to ZnSe can be reduced and the charge carrier transport thereby facilitated. With the large valence band discontinuity between BeTe and ZnSe, about 1.2 eV, or between ZnTe and ZnSe, about 0.8 eV, it is not possible to use processes for overcoming the potential barrier as are described, for example, in F. Capasso et al., Journal of Vacuum Science and Technology B 3(4) (1985), pages 1245-51, or in H. J. Gossmann et al., Critical Reviews in Solid State and Materials Science 18(1) (1993), pages 1-67.
For this reason, contact layer sequences have been proposed in which the valence band energy near the surface is gradually increased by applying semiconductor multilayers, so that the valence band edge jump is flattened and the hole barrier between ZnSe and ZnTe or between ZnSe and BeTe is lowered. For example, it has been proposed to use ZnSe/ZnTe multilayers, by means of which the valence band energy of the ZnSe within a near-surface region is raised to the valence band energy of ZnTe, and it is thus possible to produce a contact with a small Schottky barrier which has low impedance, and in particular ZnTe can be produced with high p-type conductivity (cf. WO94/15369 and Y. Fan et al., Applied Physics Letters 61 (1992), pages 3161 et seq.). This structure is referred to in the literature as “grading” or “pseudograding”. A similar contact structure employs the material BeTe instead of ZnTe in BeTe/ZnSe multilayers as a p-type contact, as described in WO94/15369, in P. M. Mensz, Applied Physic Letters 64(16) (1994), page 2148 or in U.S. Pat. No. 5,422,902. This is expected to give the contact layer an increased crystalline quality, which prevents lattice defects which are detrimental to the operation of a component. With the facility of producing lattice-matched BeTe/ZnSe “pseudograding” heterostructure contacts with good structural quality, the p-conducting electrical connection to a II-VI component can also be put onto the interface with a p-conducting substrate, as discussed in WO94/15369.
The described pseudograding contact has a complicated structure with many internal interfaces. It consists of a multilayer in which BeTe alternates with ZnSe, the layer thickness proportions of these two components being varied gradually. The total thickness of the ZnSe/BeTe contact layer sequence is between 200 Å and 1000 Å (cf. P. M. Mensz, Applied Physic Letters 64(16) (1994), page 2148 or U.S. Pat. No. 5,422,902). Over this length, the average BeTe concentration is adjusted in steps from 0 to 100% by increasing the BeTe layer thickness while at the same time reducing the ZnSe layer thickness. With the usual production parameters for BeTe and ZnSe, the total resistance of the BeTe/ZnSe contact structure is in the range from 10
−2
to 10
−3
Wcm
2
, and therefore leads to high electrical losses in the contact structure. The high resistance is attributed to the large valence band discontinuity between BeTe and ZnSe, which is 1.21 eV.
There arises another problem in the context of the contact structures with ZnSe/ZnTe or ZnSe/BeTe multilayers (pseudograding), namely when the concentrations of the dopant nitrogen are high, a large number of lattice defects are produced and make it possible for the superlattice matrix elements and the dopant to interdiffuse. Such defects can also aggregate and produce extended lattice defects in contact, which can greatly disrupt the operation of a laser diode and therefore shorten its life.
Another problem of the BeTe/ZnSe pseudograding contact is that, with the layer thicknesses used, ripples occur on the interfaces between BeTe and ZnSe which are created in order to reduce the elastic prestresses. This unevenness also has a negative effect on the functioning of a laser diode.
The transfer of holes from the valence band of, for example, BeTe to the valence band of ZnSe occurs not only in contact structures, but is also relevant, for example, in vertically emitting lasers with Bragg reflectors that contain BeTe and ZnSe. For such component structures, the use of extended grading or pseudograding is highly disadvantageous.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a II-VI semiconductor component with an improved junction from an Se-containing layer (e.g., a ZnSe layer) and a BeTe-containing layer, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type. It is a further object to provide a simple process for reproducible production of the junction.
It is a particular object to provide a contact in which the valence band discontinuity at an interface between a layer containing BeTe and a layer containing ZnSe is greatly reduced, and low-loss transfer of holes from the layer containing BeTe to the layer containing ZnSe can be achieved, in particular for p-conducting layers. The intention is especially to provide contact structures with which it is possible to produce low-loss p-type contact with p-conducting II-VI semiconductor layers that contain ZnSe.
The term materials containing BeTe is used below to denote materials such as, for example, Be
x
Mg
y
Zn
1−x−y
Te, Be
x
Cd
y
Zn
1−x−y
Te, Be
x
Mg
y
Cd
1−x−y
Te, Be
x
Mn
y
Zn
1−x−y
Te, Be
x
Sr
y
Zn
1−x&minu
Baron Thierry
Fischer Frank
Keim Markus
Landwehr Gottfried
Litz Thomas
Christianson Keith
Greenberg Laurence A.
Lerner Herbert L.
Osram Opto Semiconductors & Co. OHG
Stemer Werner H.
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