Homogenization Anneal of II-VI Compounds

1. Field of the Invention.

The present invention relates to semiconductor materials, and, more particularly, to growth of II-VI compound semiconductors materials.

2. Description of the Related Art.

Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. In particular, extrinsic p-type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane arrays operating in the 10-12 .mu.m wavelength window.

The Hg.sub.1-x Cd.sub.x Te is either formed as a bulk crystal or grown on a substrate such as CdTe by liquid phase epitaxy or other methods in order to have a more convenient substrate. Such epitaxial Hg.sub.1-x Cd.sub.x Te films typically have a thickness in the order of ten .mu.m.

Molecular beam epitaxial (MBE) films of Hg.sub.1-x Cd.sub.x Te alloys are currently grown with a substrate temperature of approximately 200.degree. C., onto a substrate which, before the growth of the Hg.sub.1-x Cd.sub.x Te film, presents a surface of CdTe. The crystal orientation of the CdTe is either (111)B or (100). Currently, the orientation preferred is the (100), which eliminates twinning which is present in the (111)B orientation. See, J. Arias et al, 5 J. Vac. Sci. Tech. A 3133 (1987). Other orientations are also used. The carrier concentration of the material is set in one of two ways. In the first, the growth conditions, that is, the substrate temperature and beam fluxes, are adjusted to give the desired carrier concentration, with the material being used in the as-grown conditions (n type is grown at about 190.degree. C. and p type at about 210.degree. C.). In the other method, which has only recently been reported, after growth the material is given an anneal in mercury saturated conditions to annihilate the acceptor states which arise from metal vacancies in the as-grown material and convert p type to n type. This anneal is performed at a low temperature, but less than 300.degree. C., for above this temperature, there is a large concentration of metal vacancies which are stable. Indeed, annealing at over 300.degree. C. will convert as-grown n type material to p type.

However, this growth procedure has the problem of producing material with anomolous electrical properties.

The present invention provides a homogenization anneal for MBE or MOCVD grown Hg.sub.1-x Cd.sub.x Te to eliminate lateral variations in x due to the growth. This solves the problem of anomolous electrical properties in the known growth procedures that are caused by these lateral variations.

The drawings are schematic for clarity.

FIG. 1 illustrates in cross sectional view Hg.sub.1-x Cd.sub.x Te grown at low temperature;

FIG. 2 is the same view as FIG. 1 but during application of the first preferred embodiment method; and

FIG. 3 is the same view as FIG. 1 but after application of the first preferred embodiment method.

A problem with the known MBE and MOCVD growth methods of Hg.sub.1-x Cd.sub.x Te on CdTe, which has not been recognized by those practiced in the art, is that surface faceting during the growth of an MBE film leads to local variations in the alloy ratio, i.e., the x value in the Hg.sub.1-x Cd.sub.x Te. For example, an MBE film growing in a (100) direction will develop surface roughness such that the surface tends to become (111)A planes. This faceting does not fully develop, however, meaning that the actual surface has different surface normals depending on position on the surface. The Hg.sub.1-x Cd.sub.x Te alloy growing beneath these different surface normals has differing x values, depending on the local surface normal; see FIG. 1 which is a cross sectional view of a Hg.sub.1-x Cd.sub.x Te film 12 grown on a CdTe substrate (not shown). Film 12 has regions 14 of high x value Hg.sub.1-x Cd.sub.x Te and regions 16 of low x value Hg.sub.1-x Cd.sub.x Te with regions 16 growing beneath facets 18 on surface 20 and adjacent regions 16 separated laterally by about 1,000 .ANG. to 3,000 .ANG. by regions 14. Regions 16 are a few hundred .ANG. thick. The magnitude of the x variation is estimated to be as much as 0.1; that is, x may be equal to 0.2 in a region 16 and equal to 0.3 in a region 14.

Thus, in the current art, material as grown does not have a uniform x-value laterally. This leads to many of the current undersirable electrical properties which these materials exhibit. Furthermore, as the tendency to facet is recognized to be a result of crystal growth, MOCVD films of Hg.sub.1-x Cd.sub.x Te grown at low substrate temperatures where interdiffusion is not large (substrate temperature less than 300.degree. C.) will also exhibit such nonuniformities in x-value.

To alleviate this problem, the preferred embodiment methods subject the MBE material to a low temperature anneal suitable to homogenize the x-value variations by lateral interdiffusion. This anneal can be performed either in-situ or after removal from the growth chamber. The amount of annealing necessary can be estimated from the formula:

where the time t is given in seconds, d is the mean spacing between regions of comparable x-value (3,000 .ANG.), and D is the interdiffusion coefficient for the alloy being grown, which may be obtained from Tang and Stevenson, 50 Appl. Phys. Lett. 1272 (1987). Necessary times at various temperatures are shown in Table I for an alloy of x=0.2. It will be noted that annealing at temperatures 300.degree. C. and lower will result in extraordinarily long annealing times. This explains why the low temperature anneal in the current art, which is generally only for a few hours, will not homogenize the grown alloy. Temperatures of 450.degree. C. and higher are also undesirable, for in addition to homogenization of the alloy, interdiffusion occurs between the substrate and the alloy film, and between a capping layer, if present, and the alloy film. Thus an anneal between 300.degree. and 450.degree. C. for a suitable time (5 minutes to 25 hours) is necessary to homogenize the film. A post anneal at a lower temperature, and under mercury saturated conditions will generally be needed to adjust the carrier concentration by generally practiced current art.

The homogenization anneal may be done in in-situ in the growth chamber, or in some other place. The mercury ambient may correspond to any point within the phase existence region at the temperature of the anneal, or with a suitable cap, such as deposited CdTe, vacuum conditions may be used.

A first preferred embodiment method of annealing an MBE-grown film of Hg.sub.1-x Cd.sub.x Te on a CdTe substrate has the following steps:

(a) After completion of Hg.sub.1-x Cd.sub.x Te film 12 growth, a 3,000 .ANG. thick film of CdTe 22 is immediately grown without withdrawal of the sample from the MBE growth chamber. The mercury pressure which was present during growth of Hg.sub.1-x Cd.sub.x Te film 12 is maintained during the early stages of growth of CdTe film 22 (to approximately 1,000 .ANG. thickness) to protect Hg.sub.1-x Cd.sub.x Te film 12 from decomposition.

(b) The sample is then moved to another chamber within the MBE apparatus, where it is annealed at 350.degree. C. for 3 hours. The annealing results in diffusion of mercury from regions 16 to regions 14 and cadmium from regions 14 to regions 16 as illustrated by the wavy arrows in FIG. 2. This homogenizes film 12 with respect to x. The sample is then removed from the MBE apparatus and capping CdTe film 22 together with the portion of film 12 at surface 20 that has been interdiffused with capping film 22 (about 5,000 .ANG.) are removed. The resultant surface 24 of film 12 is relatively planar because the anneal has effectively diffused away facets 18.

The 3 hour anneal at 350.degree. C. is more than the approximate minimum 1.25 hours listed in Table I; this excess annealing is to insure homogenization even if regions 16 are farther apart than expected or if the interdiffusion coefficient is not accurate. However, the excess annealing also increases the interdiffusion of film 12 with the CdTe substrate and with capping film 22. Thus if a 2 .mu.m interdiffusion from the CdTe substrate were taken as an upper limit, the anneal should be limited to no more than 56 hours; but this would totally interdiffuse away capping CdTe film 22 and lead to decomposition of film 12. Increasing the thickness of capping CdTe film 22 is not practical due to misfit dislocations that propagate from it into film 12; 3,000 .ANG. is about an upper limit for the thickness of capping CdTe film 22 and thus limits the homogenization anneal at 350.degree. C. to about 11 hours.

A second preferred embodiment method of annealing an MBE-grown film of Hg.sub.1-x Cd.sub.x Te on a CdTe substrate follows step (a) of the first preferred embodiment and then steps (b') and (c):

(b') The sample is then moved to another chamber within the MBE apparatus, where it is annealed at 350.degree. C. for 1.25 hours. It is then removed from the MBE apparatus and capping CdTe film 22 plus the surface interdiffused portion of film 12 are removed.

(c) The sample is annealed in a saturated mercury atmosphere at 200.degree. C. for 24 hours; this yields an n type Hg.sub.1-x Cd.sub.x Te film 12.

Various modifications of the preferred embodiment methods may be made while retaining the features of a homogenization anneal. For example, the film growth may be by metalorganic chemical vapor deposition (MOCVD) or metalorganic MBE (MOMBE); the substrate may be CdZnTe, MnZnTe, or similar II-VI compounds; the film may be Hg.sub.1-x Zn.sub.x Te, Hg.sub.1-x-y Zn.sub.y Cd.sub.x Te or similar II-VI compounds; the capping material may be Hg.sub.1-y Cd.sub.y Te with y>x or similar II-VI material; and the homogenization anneal may be in a mercury atmosphere without a capping film of CdTe or other similar material. The homogenization anneal may be used for doped or undoped films.

The invention provides the advantage of lessened anomolous carrier concentrations and mobilities through a lateral homogenization anneal.