photodetector

development of PIN ultraviolet photoelectric detector

Huang Jin, Hong Lingyuan, Liu Baolin, Zhang Baoping

(Department of Physics, Xiamen University, Fujian, 3615)

Abstract: A PIN ultraviolet detector was developed by replacing Al GaN with quaternary alloy. The structure design and fabrication process of the device are introduced in detail, and the photoelectric performance of the device is tested. The

test results show that the forward turn-on voltage of the device is about 1. 5 V, and the reverse breakdown voltage is greater than 4 V; Under the bias voltage of-5 V at room temperature, the dark current < P > is 33 Pa, the peak responsivity at 35 nm is . 163 A/ W, and the quantum efficiency is 58%.

keywords: Al InGaN/ GaN; PIN photodetector; Uv photoelectric detector

Chinese picture classification number: TN34 document identification code: a article number: 11-5868 (28) 5-669-4

development on alingan/ Ga N PIN Ultraviolet Photodetectors

HUAN GJ in , HON G Ling2yuan , L IU Bao2lin , ZHAN G Bao2ping

(Dept. of Physics , Xiamen University , Xiamen 3615 , CHN)

Abstract : Using Al InGaN instead of Al GaN as t he source film of a p hotodetector s , an

Al InGaN2based PIN UV p hotodetector was developed. It s device st ruct ure and fabrication

processing are int roduced in detail . Measurement result s show t hat it s t urn2on voltage is about

1. 5 V , and VBR > 4 V ; under - 5 V bias voltage at room temperat ure , t he dark current is about

33 pA ; t he peak responsivity can reach . 163 A/ W at 35 nm , and t he quant um efficiency is

58 %.

Key words : Al InGaN/ GaN ; PIN p hotodetector ; Ult raviolet p hotodetector

1 Introduction

GaN-based ternary alloy Al x Ga1-x N material is a direct band gap semiconductor with a continuous wavelength range

, and its

band gap varies continuously from 3. 4 V to 6. 2 V with the change of Al composition, and the band gap range corresponds to the wavelength range

of 2～365 nm, covering. As a new generation of ultraviolet detectors, Al-Gan-based wide band-gap semiconductor detectors < P > have important applications in both military and civil fields, and have attracted extensive attention at home and abroad.

At present, there are still many

problems to be solved in Al-GaN/GaN materials and device structures: (1) The lattice mismatch between Al-GaN as the active region and GaN as the

substrate leads to higher dislocation density of the epitaxial layer and larger dark current of the ultraviolet detector; (2) The activation energy of P-doped impurity Mg is very large, and its activation rate is very low. P-type Al-gan

material has wide band gap, high work function and low hole concentration, so it is difficult to obtain

good metal contact with P-type semiconductor (ohmic contact). (3) Optimal design of the structure

, such as reducing the surface light reflectivity, optimizing the thickness of the active layer

and improving the quantum efficiency of the device, so as to improve its light responsivity.

in view of these difficulties, we put forward the following improvement measures:

(1) replacing Al GaN with

Al InGaN quaternary alloy whose lattice constant and band gap can be changed independently as the I layer of the detector;

(2) growing a layer of P-type GaN

material on P-type Al InGaN material, which is used to improve the hole concentration of the semiconductor in contact with the metal layer,

which is beneficial to the formation of good ohmic contact; (3) Using Ni/ Au double layer < P > as the P electrode, a good ohmic contact between metal and semiconductor is formed.

based on the

research of Al InGaN/ GaN PIN ultraviolet photoelectric detector, this paper introduces its structural design and manufacturing process in detail, as well as the test results of its devices

.

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Semiconductor Photoelectricity, Vol.29, No.5, October 28, Huang Jin et al.: Development of Al InGaN/ GaN PIN ultraviolet photoelectric detector

2 Problems analysis and solutions

At present, ultraviolet photoelectric detectors generally adopt Al GaN/ GaN

structure. With the increase of Al component in Al GaN and the decrease of response wavelength < P >, the lattice mismatch between Al GaN and GaN becomes larger, and the stress increases < P >, which greatly limits the device performance of Al GaN/ GaN structure, especially its dark current and responsivity. However, the

band gap Eg and lattice constant of Al-InGaN quaternary alloy can be changed independently, which makes it possible for us to

adjust the band gap to the required value and keep the dislocation density lower than

, thus reducing the dark current. The broken line generation

in Figure 1 shows the range of band gap width

of Al InGaN whose lattice constant is consistent with that of GaN. If Al InGaN quaternary alloy is used as the active region, the problem caused by lattice mismatch can be solved.

the relationship between lattice constant a of quaternary alloy Al x Iny GazN and composition can be expressed as [3]

AAL x in y gazn = xaaln+yainn+zagan (1)

where x+y+z = 1. The relationship between the band gap of Al x Iny GazN and the composition can be expressed as [4 ]

Q( x, y, Z) = xy T12

1-x+y

2+yzt23

1-y+z

2+

xzt13

1-x+z

2/(xy+yz+xz) (2). Tij (α) = α bj+(1-α) Bi+Bij α (1-α), I, J = < P > 1, 2, 3 represent AlN, InN ,GaN ,B represent the < P > forbidden band width of binary alloy, B represents the bending coefficient of ternary alloy, and b12 =-5;

b23 = - 4. 5 ; b13 = - 1 。

fig. 1

relationship between band gap width and lattice constant of wurtzite-structured GaN-based materials

if aAl x In y Ga zN = aGaN, that is, Al InGaN is lattice matched with gan crystal

. The parameters in table 1 are brought into equation (1), and x ∶y

= 4. 47 ∶1 is obtained. Then, the

band gap width of Al InGaN lattice matched with GaN ranges from 3. 39 eV (GaN) to 4. 67 eV

(Al. 817 In. 183N), and the corresponding wavelength ranges from 365 nm (GaN) to

266 nm (Al. 817 In. 183 N). This band is just in the solar blind < P > region and is an ideal detection band for ultraviolet detectors.

table 1 band gap and lattice constant of wurtzite-structured GaN-based materials [2]

parameters gan AlN inn

a/nm .318 9.311 2.353 3

c/nm .518 6.498 2.569 3

eg/ EV 3. 39 6. 2 1. 9

3 experimental results and analysis

3. 1 sample structure growth and material properties

In this study, Al InGaN materials grown by MOCVD

system used by China Academy of Sciences are used. Sample a is the overall structure of

PIN ultraviolet photoelectric detector developed by us. Firstly, GaN buffer layer was grown on the bottom of Al2 O3 liner

, then 3 μm Si-doped n2

GaN was grown, then . 2μm undoped i2Al InGaN was grown, then . 2μm Mg-doped p2Al InGaN was regenerated, and finally . 1

μm Mg-doped p2GaN was grown as ohmic contact layer. In order to study the

properties of undoped Al InGaN layer and P-type Al InGaN layer in the middle of

, we have grown sample B and sample C respectively. Sample B

firstly grows GaN buffer layer on Al2 O3 substrate, then grows 3

μm Si-doped n2GaN, and finally grows . 1μm undoped i2

Al InGaN. Sample c grows GaN

buffer layer on Al2 O3 substrate, then grows 3μm Si-doped n2GaN, and finally grows . 1

μm Mg-doped p2Al InGaN.

X-ray triple crystal diffraction experiments were carried out on samples B and C, respectively. Figures

2 (a) and (b) are the X-ray triple crystal diffraction spectra of samples B and C, respectively.

the peak of 34. 565 in fig. 2 (a) is GaN (2) and the peak of 34. 62

is Al InGaN (2). The 34. 565

peak in fig. 2 (b) is GaN (2) peak, and the 34. 583

(2) peak is alingan.

thus, the calculated lattice constants of samples b and c are listed in table 2

. It can be seen from the calculation results that the lattice constants of

Al InGaN and GaN in sample B and sample C are basically matched.

(a) Sample B

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Semiconductor Opto Electronics Vol.29 No.5 Oct.28

(b) Sample C

X-ray triple crystal diffraction spectrum of the sample in Figure 2

Table 2 Lattice constants of samples B and C

Sample cGaN/nm cAl InGaN/ Nm

δ c

cgan

/%

b.518 5.518 .96

c.518 5.518 31.37

In order to analyze the composition of Al InGaN materials, the samples were measured by

PL spectrum. Comparing fig. 3 (a), fig. 3 (b) and fig. 3 (c)

, the emission peak of 358. 6 nm is the band-edge emission of p2Al InGaN; The emission peak of

365 nm is the band-edge emission of GaN; The

emission peak of i2Al InGaN basically coincides with that of GaN. It is calculated that the forbidden band width Eg = 3. 46 eV for p2

Al InGaN.

(c) Sample A

PL spectrum of the sample In Figure 3 at room temperature (3 K)

According to the above analysis, the ratio of Al component to In component in the

Al-X-Y-GaZn material matched with GaN lattice is

4. 47 ∶1, so we can determine that the group of p2Al InGaN materials

can be divided into Al.

3. 2 device technology

in this paper, a PIN

photodetector with the structure shown in fig. 4 was prepared by conventional technology. It includes n2GaN bottom layer, i2Al InGaN light absorption

receiving layer, p2Al InGaN transition layer and p2GaN ohmic contact layer.

SiO _ 2 is used as the protective layer and antireflection film of the device, and Ti/ Al/ Ni/

Au is used as the N electrode and Ni/ Au is used as the P electrode.

fig. 4 schematic diagram of alingan/ganpin structure

the process of p-type ohmic contact was optimized in the experiment, and the material

was K299 p-type sample. The alloy temperature optimization shows that the contact performance obtained by

at 5℃ is the best, and the specific contact resistance is 1. × 1-2

ω cm2. Subsequently, electrodes were fabricated on the samples of K299 (p2GaN) and

K294 (p2Al InGaN), respectively. As a result, the I2V characteristics of P-type

Al InGaN were very poor and the resistivity was very high, so it was difficult to form

ohmic contact. In contrast, the I2V characteristics of P-type GaN are much better < P >, and an ohmic contact is formed. So we grow a layer p2 on the P-type

Al InGaN layer.