II-VI Semiconductor Materials and their Applications

MariaC. Tamargo, Maria C. Tamargo

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9780367849009
• eBook ISBN:
  9781351439367
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

II-VI Semiconductor Materials and Their Applications deals with II-VI compound semiconductors and the status of the two areas of current optoelectronics applications: blue-green emitters and IR detectors. Specifically, the growth, charactrtization, materials and device issues for these two applications are described. Emphasis is placed on the wide bandgap emitters where much progress has occurred recently.The book also presents new directions that have potential, future applications in optoelectronics for II-VI materials. In particular, it discusses the status of dilute magnetic semiconductors for mango-optical and electromagnetic devices, nonlinear optical properties, photorefractive effects and new materials and physics phenomena, such as self-organized, low-dimensional structures.II_VI Semiconductor Materials and Their Applications is a valuable reference book for researchers in the field as well as a textbook for materials science and applied physics courses.

Chapter 1

HgCdTe Growth and Detector Technology

Dennis Edwall and Jagmohan Bajaj

Rockwell Science Center, Thousand Oaks, California, USA

1 Introduction

2 PACE- 1 Materials and Devices

3 MBE HgCdTeICdZnTe Materials and Devices

3.1 Growth and Characterization

3.2 Devices

4 Multiplexers

5 FPA Performance

6 Summary

Acknowledgments

References

1 INTRODUCTION

The infrared (IR) spectrum for imaging applications is nominally separated into the short wavelength (SWIR, ~1–3 μm), mid-wavelength (MWIR, ~3–5 μm), and the long wavelength (LWIR, 8–14 μm) spectral bands. Although many types of IR sensors exist, HgCdTe currently offers the highest performance. Applications include night vision, military surveillance, earth sensing and biomedical sensing. This chapter covers HgCdTe material properties, detector fabrication and performance, and focal plane array (FPA) performance for each of these bands.

The hybrid focal plane array is made up of two separate components: a detector array and a readout integrated circuit (ROIC) multiplexer. The HgCdTe detector array consists of photovoltaic diodes processed in epitaxially grown material on a suitable substrate. Two types of material growth techniques are being pursued at Rockwell. Liquid phase epitaxy (LPE) is used to grow HgCdTe on lattice-mismatched CdTe/sapphire (PACE-1) substrates and covers the SWIR and the MWIR IR bands. Molecular beam epitaxy (MBE) is used to grow HgCdTe on lattice-matched CdZnTe substrates and covers all IR bands. When there is a choice of substrate material, the trade-off between performance and cost dictates the selection. The other component of the FPA, the ROIC, converts the photoinduced current from the detector array into a voltage. The ROICs are fabricated at an existing commercial Rockwell silicon foundry. Finally, the hybrid FPA is fabricated by depositing indium columns onto the detector and the ROIC, and mating the two devices through cold welding. Figure 1 shows a cross-section of a hybrid focal plane array. The FPA is backside illuminated through the IR-transparent substrate (sapphire is transparent from the visible to 5.5 μm, CdZnTe from 0.9 to 20 μm).

Figure 1 Cross-section of a hybrid focal plane array, backside illuminated through the substrate.

Epitaxial HgCdTe is usually prepared using LPE, MBE or metal-organic vapor phase epitaxy (MOVPE) [1]. For MBE, most growth uses solid source Te and CdTe, but metal-organic sources have also been used [2]. Of these approaches, LPE is the most mature technology. While LPE layers may be grown from Hg-rich or Te-rich solutions, work at Rockwell has concentrated on growth from the Te comer. However, the results below focus on MBE growth due to its inherent advantages of composition and doping control for complex multilayer structures.

This chapter is structured as follows: Section 2 covers PACE-1 (CdTe/sapphire) LPE HgCdTe materials and detector technology for SWIR and MWIR spectral regions. Section 3 covers lattice-matched CdZnTe substrate-based MBE HgCdTe materials and detector technology for SWIR, MWIR and LWIR spectral regions. Section 4 provides ROIC status. Section 5 highlights FPA performance for various IR bands. Section 6 concludes with a summary of IR technology status and near-term future trends.

Figure 2 Twenty-one 256 × 256 detector arrays fabricated on a 3 in. diameter PACE-1 substrate

2 PACE-1 MATERIALS AND DEVICES

Rockwell has pioneered the LPE growth of HgCdTe on sapphire substrates. This technology is called PACE-1 which means producible alternative to CdTe for epitaxy [3]. The sapphire substrate provides advantages in terms of relatively large area (3 in. diameter is now standard size) compared to other IR substrate materials, durability during processing and reliability to thermal cycling. The advantages of large area substrates become evident in Figure 2 which shows that 21 (40 μm pitch) 256 × 256 detector arrays can be fabricated on a single 3 in. diameter substrate. Similarly, 28 (30 μm pitch) 320 × 240, 8 (27 μm pitch) 640 × 480 or 5 (18.5 μm pitch) 1024 × 1024 detector arrays can be fabricated simultaneously on a single 3 in. diameter substrate.

The MCT process line in Thousand Oaks, California, is in continuous production of 256 × 256 HgCdTe detector arrays fabricated on 3 in. diameter PACE-1 substrates. The sapphire substrates [(0001) orientation] are prepared by coating them with a thin buffer layer of CdTe using the MOVPE process. LPE HgCdTe with the (111)B orientation is subsequently grown on these hybrid CdTe/sapphire substrates using Te-rich melt solutions and a tipping technique [4].

Computer-controlled and -monitored LPE furnaces are equipped to grow HgCdTe simultaneously on two 3 in. diameter PACE-1 substrates. Figure 3 is an example of the reproducibility of materials parameters for over 200 MWIR HgCdTe growth runs. Figure 3a and b shows the average and the standard deviation, respectively, of the room-temperature cut-on wavelength measured at five locations on the wafer. The data is plotted as a function of the LPE furnace number over consecutive growth runs.

Figure 3 Run-to-run reproducibility of LPE grown MWIR HgCdTe room temperature cut-on wavelength.

Figure 4 Schematic of an n-on-p device structure fabricated on PACE-1 substrate.

The device structure for PACE-1 technology is an n-on-p photodiode shown schematically in Figure 4. Planar-mesa detector pixels are formed by boron ion implantation in Hg vacancy-doped LPE HgCdTe [5,6].

Device processing is carried out in individual lots of 6 3 in. wafers, and as shown in Figure 2, each 3 in. wafer contains 21 256 × 256 detector arrays with 40 μm center-to-center pixel spacing. Each detector die is surrounded by test strips, each containing 20 diodes identical to the array pixels. These test strips are cryoprobed to select detector die for hybridization to the ROIC. The mean value of the cryoprobe zero-bias resistance-area product (R₀A) for the entire wafer (23 test strips; 460 diodes) for approximately 50 consecutive wafers processed is shown in Figure 5.

The quantum efficiency (QE) at 77 K is typically near 80%. The spectral cutoff (50% point) is approximately 5.0 μm at 77 K. The data show that the baseline n-on-p MWIR technology with PACE-1 substrates meets the flowdown specifications of several current applications that require operation under high background conditions.

While sapphire substrates have advantages of size, mechanical rigidity, inertness, availability, relatively low cost and relatively good thermal matching to both HgCdTe and the Si ROIC, their main disadvantages are nontransparency beyond 5.5 μm and large lattice mismatch to HgCdTe (~ 14%). The large lattice-mismatch results in relatively high HgCdTe dislocation densities (> 1 × 10⁶ cm⁻²). While this has been shown [7] not to be a serious issue for the SWIR and MWIR applications, it is for LWIR. Therefore, the more expensive and fragile CdZnTe substrates are the substrates of choice for LWIR applications for both LPE and MBE growth.

Figure 5 R₀A average over a 3 in. diameter wafer for MWIR PACE-1 HgCdTe diodes for 50 consecutively processed wafers. Each data point represents an average from 23 test strips (total of 460 diodes) uniformly distributed over a 3 in. diameter wafer.

3 MBE HgCdTe/CdZnTe MATERIALS AND DEVICES

Rockwell also pioneered the development of MBE growth of HgCdTe on lattice-matched (211)B CdZnTe (3.5 ± 1% Zn) substrates for high-performance applications [8–12]. MBE is one of the leading growth techniques in semiconductor technology for developing new device structures, due to its relative simplicity and precise control over the growth parameters. For HgCdTe, it offers great flexibility in device design, as the growth is carried out at relatively low temperatures (<200°C), allowing synthesis of multilayer structures with abrupt interfaces. Developments in MBE HgCdTe growth and doping control have led to the demonstration of high-performance FPAs for imaging applications over a broad spectral range. These developments include significant work, both experimental [2,1...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Introduction
• 1 HgCdTe Growth and Detector Technology
• 2 Blue-Green Semiconductor Lasers
• 3 Molecular Beam Epitaxy of Wide Gap II-VI Compounds
• 4 Optical Properties and Electronic Structure of Wide Band Gap II-VI Semiconductors
• 5 Nonlinear Optics and Propagation of Excitons
• 6 BeryIIium Containing II-VI Compounds
• 7 II-VI Materials for Visible Light Emitters
• 8 Spin Engineering in II-VI Magnetic Semiconductor Heterostructures
• 9 Self-Organized Low-Dimensional II-VI Nanostructures
• Index