Arsentiev I. N.1, Bobyl A. V.1, Boltovets N. S.2, Ivanov V. N.2, Konakova R. V.3,

Kudryk Ya. Ya.3, Lytvyn O. S.3, Milenin V. V.3, Tarasov I. S.1, Belyaev A. E.3, Rusu E. V.4 11offe Physical-Technical Institute of the Russian Academy of Sciences 26, Polytekhnicheskaya St., Sankt-Peterburg – 194021, Russia Tel.: + (7-812) 247-91-34, e-mail: tarasa@hpld.ioffe.rssi.ru 2State Enterprise Research Institute “Orion”

83, Eugene Pottier St., Kiev – 03057, Ukraine Tel.: + (380-44) 465-05-48, e-mail: bms@i.kiev.ua 3V. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine

41, Nauki Prospect, Kiev – 03028, Ukraine Tel.: + (380-44) 265-61-82, e-mail: konakova@isp.kiev.ua 4 Institute of Applied Physics of the Academy of Sciences of Moldova 5, Academia St., Kishinev – MD-2028, Moldova Tel.:+ (3732) 73-90-48, e-mail: rusue@lises.asm.md

Abstract – A new technological approach to production of structurally perfect epitaxial films LPE-grown on “soft” porous /?+-lnP substrates is considered. We studied surface morphology, boundary between phases in TiBx-n-lnP contact and l-V curves of Au-TiBx-n-lnP Schottky diodes made on “soft” and “rigid” (standard) n+-lnP substrates. The advantages of epitaxial layers grown on porous n+-lnP substrates and barrier structures on their basis are demonstrated.

I.  Introduction

The development of microwave (mm wavelength range) devices imposed severe requirements on semiconductor epitaxial structures that served as a basis for device manufacturing. As a rule, such epitaxial structures are thin (2-3 epitaxial layers of total thickness below 1 |im) and are characterized by a high level of intrinsic stresses. In this case the active areas of microwave devices (especially of those with Schottky barriers) is located in a thin near-surface layer immediately adjacent to a metal-semiconductor contact and is sensitive to structure changes in this layer due to both technological (during device manufacturing) and operational factors. Therefore the intrinsic stresses in these structures must be reduced to such an extent as to practically exclude stress relaxation induced by external actions.

So, one of the basic problems is the development of new methods for production of epitaxial structures with low (or even zero) level of stresses [1, 2]. Fulfillment of the main requirement – obtaining structurally perfect epitaxial layers in the film-substrate structures without stresses still meets difficulties. One of the ways of solving this problem (that is being developed at Ioffe Physi- cal-Technical Institute) is formation of epitaxial layers on “soft” (porous) semiconductor substrates [3].

In this work we studied the epitaxial layers and Schottky barriers on the basis of n-type indium phosphide grown on porous n+-lnP (100) substrates.

II.    Sample preparation and experimental procedure

Three-layer n-lnP films were LPE-grown (see our earlier work [4]) on specially prepared InP (100) substrates doped with tin (2-1018 cm’3). Porous material interlayers (9-12 |im thick) were obtained using electrochemical etching in chloride and bromide water solutions. Epitaxy was performed from an InP—In solution- melt; crystallization began at a temperature of 655 °C. To suppress primary underetching and liquid phase penetration into pores, the initially saturated melt was overcooled by 10 °C before contacting with the substrate [5]. For the sake of comparison, epitaxial layers on the standard “rigid” substrates were grown also in the same process.

The Schottky barriers were formed using magnetron sputtering of TiBx from a target of stoichiometric composition followed by gilding. Ohmic contacts were prepared with Au-Ge eutectics. The diode structures with diameters of 20…200 |im (at 20 |im intervals) were made with photolithography.

We studied the component concentration depth profiles in TiBx-lnP barrier layers for structures of both types with Auger electron spectroscopy; surface morphology of epitaxial layers was studied with atomic force microscopy. Besides, we took l-V curves of the diode structures.

III.  Results of measurements

An analysis of surface morphology of epitaxial layers grown on substrates of both types showed that these surfaces are not too different from each other. They are characterized by Gauss distribution of irregularity heights and presence of grains whose boundaries were not clearly pronounced. The distinctions were in size and boundary depth (Table 1).

Table 1. Surface characteristics of epitaxial layers grown on “soft” and “rigid" InP substrates

Characteristics

"Soft"

substrate

"Rigid"

substrate

surface grain size, +2 nm

33

52

surface fragment (5×5 |im2) height, nm

6.877

47.042

surface irregularity size, nm

0.2

0.9

A dramatic increase of surface irregularity size and height on a “rigid” substrate indicates at its considerable surface nonuniformity as compared with that for a “soft” substrate.

The component concentration depth profiles in TiBx-lnP barrier structures (Fig. 1) evidence that mass transfer in the TiBx-lnP contacts formed on “soft” InP substrates is proceeding slower than in similar contacts formed on “rigid” substrates. These results agree with those of x-ray diffractometry [6] that characterize a device structure on a “soft” substrate as having intrinsic stress 40% below that on a “rigid” substrate.

The results of our measurements of l-V curves in the di- ble 2. They are in agreement with the data on structure ob- ode structures of both types and calculations of Schottky tained when studying surface morphology of epitaxial layers barrier heights дь and ideality factors n are presented in Та-and component concentration depth profiles in the contacts.

Table 2. <