Characterization of PbTe p - n+ junction grown by molecular beam epitaxy

Abstract

In this work we investigate the electrical properties of PbTe p - n+ junction. Mesa diodes were fabricated from p - n+ PbTe layers grown on (111) BaF2 substrates by molecular beam epitaxy. From the analysis of the current versus voltage characteristic measured at 80K, the incremental differential resistance and the series resistance were determined. The capacitance versus voltage curves were measured at a frequency of 1MHz. The one-sided abrupt junction was checked through the 1/C²xV plot. From the linear fit, the hole concentration and the depletion layer width in the p-side were obtained. The high detectivity values measured for the p - n+ PbTe diode confirm that it is very suitable for infrared detection.

Characterization of PbTe p – n⁺ junction grown by molecular beam epitaxy

A.S. Barros; E. Abramof; P.H.O. Rappl; A.Y. Ueta; H. Closs

Laboratório Associado de Sensores e Materiais - LAS, Instituto Nacional de Pesquisas Espaciais, INPE, CP 515, 12245-970, São José dos Campos-SP, Brazil

ABSTRACT

In this work we investigate the electrical properties of PbTe p – n⁺ junction. Mesa diodes were fabricated from p – n⁺ PbTe layers grown on (111) BaF₂ substrates by molecular beam epitaxy. From the analysis of the current versus voltage characteristic measured at 80K, the incremental differential resistance and the series resistance were determined. The capacitance versus voltage curves were measured at a frequency of 1MHz. The one-sided abrupt junction was checked through the 1/C²xV plot. From the linear fit, the hole concentration and the depletion layer width in the p-side were obtained. The high detectivity values measured for the p – n⁺ PbTe diode confirm that it is very suitable for infrared detection.

1 Introduction

The lead salts are appropriate materials for infrared detection applications [1]. Due to its cutoff wavelength l_c=5.9 mm at 77K, PbTe is an interesting option for the mid-wavelength infrared region. PbTe one-sided abrupt p – n⁺ junctions have been proved to work properly as infrared sensors [2-4]. The detector performance strongly depends on the electrical characteristics of this junction.

In this work, we investigate the electrical characteristics of mesa diodes fabricated from p – n⁺ PbTe layers grown by molecular beam epitaxy on (111) BaF₂ substrates. For this purpose, dedicated current versus voltage (IxV) and capacitance versus voltage (CxV) measurement systems were employed, tested and calibrated with commercially available devices. Both systems were computer controlled through an IEEE-488 interface and we have developed the control and measure programs in Visual Basic platform.

2 Sample preparation

The samples were grown on freshly cleaved (111) BaF₂ substrates by molecular beam epitaxy in a RIBER 32P MBE apparatus equipped with PbTe, Te and Bi₂Te₃ effusion cells [5]. The beam equivalent pressure (BEP) originating from each effusion cell was measured separately in a Bayer-Alpert flux monitor, and was used as the main growth parameter. A 12 keV reflection high-energy electron diffraction (RHEED) system was used to evaluate in situ the growth conditions. During this experiment, the growth temperature was always 300^oC. Before growing the actual p – n⁺ structures, reference epitaxial layers were grown on BaF₂ substrates and characterized by Hall effect measurements, in order to obtain the desired carrier concentration for each layer in the junction. The PbTe flux was about 7x10^–7 Torr, leading to a growth rate of 2.2 Å/s. The p-type layer, with a hole concentration of approximately 1x10¹⁷ cm^–3, was obtained using an additional Te flux to the PbTe main beam in a ratio of 1:10. Keeping this PbTe flux but using an additional Te flux in ratio of 1:50, a series of Bi doped PbTe epitaxial layers were grown with Bi₂Te₃ effusion cell temperatures varying from 370 to 450^oC. In these conditions, the electron concentration at 77K varied from 1x10¹⁷ to 4x10¹⁹ cm^–3, as determined by Hall measurements. For all these layers, the growth started with an island nucleation, evidenced by a spotty RHEED pattern. After 0.5 to 1 min (600 to 1200Å), the initial islands coalesced and the RHEED pattern changed to a streaky one. As the layer thickness reached approximately 0.5 mm, the RHEED pattern already showed elongated spots lying on a semicircle, characteristic of an atomically flat surface. This RHEED pattern persisted until the end of growth, and no surface reconstruction was observed.

The p – n⁺ junction sample was then formed by growing the first p-type PbTe layer (6 mm thick and ~ 1x10¹⁷ holes/cm³) followed by the 2 mm thick n⁺ PbTe layer with electron concentration of 1x10¹⁹ cm^–3. In order to electrically characterize this junction, mesa structures, as shown in Fig. 1, were etched in the samples. Au discs with 200 nm of thickness, as monitored by a quartz crystal oscillator, were deposited on the n⁺ layer from an electron beam source through a stainless steel mask with holes of 0.3 mm in diameter. These Au discs were used as protective masks for the etching as well as metallic contacts. The mesas were etched with a Br₂:HBr:H₂O (1:40:40) solution. Additional gold pads were deposited on the p-side. Gold wires were soldered to both Au pads with indium. The diode was mounted on a Cu plate and assembled in a LN₂ cryostat.

3 Electrical characterization

The resistivity, carrier concentration and mobility of the reference layers were measured at 300 and 77K in a automatic data acquisition Hall Effect system with an electromagnet field of 0.7 Tesla.

To measure the current versus voltage (IxV) characteristic of the diode, a dedicated system composed by a programmable power supply (Keithley 220), an ammeter (Keithley 2010) and a voltmeter (Keithley 199) was employed using a special cable assembly which guarantees accurate measurement of the exact voltage drop across the junction and of the actual current flow. The system is computer controlled through an IEEE-488 interface and we have developed the IxV control and measurement program in a Visual Basic platform. The IxV curves of commercial Si diodes were used to test and calibrate the system.

Figure 2 (upper panel) shows the IxV curve of the PbTe p – n⁺ junction at 80K. This is a typical IxV curve for PbTe junctions, showing a leakage resistance in the reverse bias condition and a series resistance in the forward direction. The graph in the lower panel of Fig. 2 shows the inverse derivative of the IxV curve. A value of 950W for the incremental differential resistance at zero bias (R₀) was directly obtained from this plot, leading to a R_oA product of 2.3W.cm², which is approximately one order of magnitude lower than those found in the literature [2]. A relatively higher dislocation density in the PbTe layer may be responsible for this behavior. This fact is also in agreement to the relatively low saturation mobility (at low temperatures) in our p-type layer as compared to the values found in the literature. The IxV curve shown in Fig. 2 also indicates that the series resistance observed in our diode tends to a value of less than 100W.

To further characterize the p – n⁺ diode, CxV curves were measured with a 1 MHz HP4280A capacitance meter. This equipment was also controlled via an IEEE-488 interface and a measurement program was developed in Visual Basic. A background capacitance which includes the cables and the cryostat with open contacts was measured as a function of applied voltage and was subtracted from each CxV measurement. Fig. 3 shows the 1/C² versus reverse voltage plot for our p-n⁺ diode, where C is the capacitance per unit area. Its almost linear behavior indicates that a one-sided abrupt junction was formed.

For this type of junction, the slope of the 1/C²xV plot is given by:

d(1/C²)/dV = 2/(qke₀p),

where q is the electron charge, k is the dielectric constant, e₀ is the vacuum permittivity and p is the hole concentration. From the slope of the solid line in Fig. 3 and using k = 400 for PbTe, a value of p = 5×10¹⁶cm^–3 was obtained, which is close to that measured by Hall effect. The depletion width W can be also obtained by the CxV characteristic through

W(V) = ke₀/C(V).

For zero bias (V=0), the depletion width for our junction turned out to be 0.5 mm, increasing to 0.77 mm as the intensity of the reverse voltage rised to 0.40V.

In order to test the fabricated device as an infrared detector, the integral detectivity D^* of the p-n⁺ diode was measured at 80K (FOV=90^o). A blackbody source (T=900K) illuminated the detector through the BaF₂ side with a power density of 565 mW/cm² through a chopper with frequency of 900Hz. The signal to noise ratio measured in a lock in amplifier with a bandwidth of 14 Hz was 8400, leading to an integral detectivity D* of 1.1x10⁹ cmHz^1/2W^–1. This value indicates that the p-n⁺ PbTe diode is very suitable for infrared detection.

4 Conclusions

p – n⁺ PbTe junctions grown by MBE on BaF₂ were electrically characterized. From the IxV curve, the R_oA product and the series resistance were determined. The 1/C²xV plot suggested that a one-sided abrupt junction was formed. The hole concentration and the depletion layer width of the p-side were determined by linear fitting. Despite the relatively small R_oA product measured, the p-n⁺ PbTe diodes exhibited high detectivity D^* values, and confirmed to be very appropriate for infrared detection application.

Acknowledgement

We thank the partial support of FAPESP (Grant Nr. 00/12529-6) and CNPQ (302395/02-4).

Received on 31 March, 2003

• [1] A. Rogalski, Infrared Physics & Technology 43, 187 (2002).
• [2] J. John and H. Zogg, J. Appl. Phys. 85, 3364 (1999).
• [3] D. Zimin, K. Alchalabi, H. Zogg, Physica E 1220 (2002).
• [4] C. Boschetti, P.H.O. Rappl, A.Y. Ueta, and I.N. Bandeira, Infrared Phys. 34, 281 (1993).
• [5] P.H.O Rappl, H. Closs, S.O. Ferreira, E. Abramof, C. Boschetti, P. Motisuke, A.Y. Ueta, and I.N. Bandeira, J. Cryst. Growth 191, 466 (1998).

Publication Dates

• Publication in this collection
  31 Aug 2004
• Date of issue
  June 2004

History

• Received
  31 Mar 2003