LOW TURN-ON VOLTAGE AIGaInP LEDs USING THERMALLY EVAPORATED TRANSPARENT CONDUCTING INDIUM-TIN-OXIDE (ITQ Y. H. Aliyu, D.V. Morgan, H. Thomas a n d S. W. Bland+ Electronics Division, Cardiff School of Engineering, University of Wales Newport Road, P. 0.Box 917 Cardiff U. K CF2 1XH +Epitaxial Product International St. Mellons Cardiff U. K
ABSTRACT Low tum-on voltage AlGaInPlGaInP surface emitting LEDs have been achieved using a thermally evaporated transparent conducting Indium Tin Oxide (ITO) layer. The devices have lower forward series resistance (1-3) Ohms compared to standard AulZn devices which have a series resistance greater than 5 Ohms. The ITO/AlGaInP LEDs emit orange light, with a peak wavelength of 600 MI and full width at half maximum (FWHM) of 15 nm. A forward voltage of typically 1.70 V at 20 mA was obtained. Variations in the thicknesses of the cladding and GaAs cap layer thicknesses did not cause any significant change in the device tum-on voltages. Evidence of reduced junction heating has been observed. 1.0 INTRODUCTION
figh-brightness visible light emitting &odes(LEDs) are very attraCtive as light sources for mobile indicators and outdoor display applications such as High Mounted Stop Lamps (HMSLs) and traffic lights. The ( ~ x G ~ ~ - x ) O quaternary . ~ ~ . ~ Palloy lamce matched to GaAs is the most important candidate because of its direct band gap, extendmg up 2.3 eV as the a l u " composition x increases from 0 to 0.7. AlGaW LEDs can be used for applications that require either high output power or low power consumption. LEDs with light output, which are vastly superior to aII the existing technologies have recently been demonstrated using this quaternary alloy [ 11. Two key issues are the growth of a thick, low-resistive, high bandgap layer to act as a window material and to spread the current below the opaque metal contact. Other problems include that of internal reflection and substrate absorption losses, whch limit the external quantum efficiency of these LEDs. Therefore to maximise the light extraction from the active layer, it is necessary to incorporate a wide band-gap material with sufficient th~chessand hgh enough electrical conductivity on top of the DH layers to spread the current from the opaque top metal contact. The use of transparent conducting h&um-tin-oxide (KO)as a contact materid seems promising owing to an interesting combination of properties, namely hlgh visible transparency and hgh electrical conductivity. IT0 films have been used previously as window material for both Lasers [2,3J and LEDs[4].
This paper describes a detailed investigation on the use of thermally evaporated transparent conducting IT0 material as a window material and current spreadmg layer on surface emitting AlGaInP/GaInP LEDs. Details of the electrical and optical properties the IT0 layers are reported elsewhere. CurrentVoltage (I-V) characteristics, Light-Current (L-I)>Light-Voltage(L-V), and Spectral Response (S-R) were used to characterise the LEDs.
0-7803-2537-0/95/$4.00 @ 1995 IEEE
82
.
2.0 EXPERIMENTAL DETAILS
AlGaInP/GaInP layers were grown on n+-GaAs substrates by low pressure Metal Organic Chemical Vapour Deposition (MOCVD)by Epitaxial Products International Ltd, St. Mellons, Cardiff UK. The devices consist of a quaternary undoped active region bounded by p and n-type AlGalnP cladding layers doped with Zn and Si respectively. A thin (0.02 pn ) p-type GaInP layer with a doping of 1 d 8 is grown on the p-type cladding layer. The thiclcness of the upper p-type cladding layer was varied from 0.5 pto 5.0 pn. The hghly doped p-type GaAs cap layer thickness was varied from 0 . 0 3 to~ 0.05~ m in order to determine its optimum value. A transparent conducting IT0 layer w a y deposited on the DH layers to expand the light emission area and hence improve the light extraction by acting as a window layer. The IT0 used for the evaporation is composed of Indium oxide (In02)with 10 mol% tin Oxide(SnO2). High conductivity IT0 layers were obtained by thermally evaporating high purity IT0 material in a high vacuum (lo-' torr) under an oxygen partial pressure of lo4 torr at room temperature. The combination of excellent conductivity lo5 mS, high transparency ( > 90 %) and a lower refiactive index (n = 2. l), compared with that of semiconductor layers leads to an improvement in CUKent spreading and an improvement in light extraction from the active region by providing better rehctive index matching at the semiconductor/air interface. Metal bondmg contacts were fabricated on the LEDs by depositing Au-Zn on the IT0 and alloyed Au-Ge (NI) contact to the n-type GaAs substrate annealed at 450 "C for 3.5 minutes in fonning gas. The final device structure is shown schematically in Figure (2). The sizes of the LEDs are 3 0 0 p x 3 0 0 p with a 100 pn Au-Zn dot, which is also used for external connection. 3.0 RESULTS AND DISCUSSIONS The LEDs exhibit a normal p-n diode behaviour, at a given bias level, the total current is the sum of a (radiative) diffusion current Id and a (non-radiative) recombination I,, which is mostly donmated by surface current near the perimeter of the LED chip. Similar Current-Voltage (I-V) characteristics were obtained for both the ITO/AlGaInP LED and the standard AlGaInP LED without the IT0 film.Figure (2) shows the Current-Voltage Characteristics (I-V) of both the standard LED and lTO/AlGaInP LED. A typical value of the saturation current density of the order of J = 6.6 x (Nan2)with a series resistance between 1-3 Ohms was obtained for the ITO/AlGaInP LEDs. The standard LEDs are @/an2) and a series characterised by a slightly lower current densities typically J = 8.0 x resistance greater than 5.0 Ohms . A small series resistance is most important for decreasing joule heating effect and thus increasing the quantum efficiency of the LEDs. The diode ideality factors in the radiative region and the non radiative regions are n = 2.1 and n = 2.7 respectively. Capacitance-Voltage (C-V)characteristics of the ITO/AlGaInP LED were measured. Typically the zero bias capacitance of the diodes are about 269.0 pF. Figure (3) shows Light-Voltage (L-V) characteristics of both ITO/AlGaInP LED and the standard LED. The ITO/AlGalnp LEDs are tumed on at 1.68 V and exhibit a typical forward voltage (VF) of 1.70V at 20 mA and up to 1.85 V for the standard devices. Figure (4) compares the L-V characteristics for all the combinations of cladding layer and GaAs cap layer thicknesses( Table 1 ). Devices D1 and D2 have exactly the same structure but different sizes. They are 500 pn x 500 p and 300 pn x 300 p and are represented on the table as D. Variations in the thicknesses of the cladding and GaAs cap layers have not caused any significant change in the device tum-on voltages. Figure (5) shows light output intensity versus dc current characteristics for all the Combinations of cladding layer and GaAs cap layer thicknesses ( Table 1 ). Reducing the thickness of the highly doped p+-GaAs cap layer from 0.05 to 0.03 jm causes a reduction in the light intensity by more than 50 %. The reasons for this reduction is not fully understood, but shows that the cap layer plays a role in determining the effectiveness of the IT0 film as a current spreading layer. Increasing the thickness of the p and n-type AlGaInP cladding layers from 0.5 to 1.Owimproved the light intensity by up to 25%.
A significant improvement of light intensity was achieved by increasing the p-type AlGaInP cladding layer from 1.O to 5 .O p.This increment is attributed to the enhancement of emission through the sides of a thick window layer. Total light output in LEDs has been shown to be a function the window thickness. If the window is too thin some of the light will be totally internally reflected from the top surface of the device and down into the absorbing substrate. cladding layers produced the best Devices with 5.0 pm p-AlGaInP and 1.0 pm n - A " performances. The light intensity of ITO/AlGalnp LED is about 10% better than that of the standard LEDs. In general, for most of the devices studied in this work, the light intensity of the ITO/AlGaInP compare favourably to those of the standard LEDs. The light intensity increases linearly wth current, no evidence of sub linearity of the intensrty curve or radiance saturation was observed at hgher currents ( up to 100 mA). Visual inspection of the ITO/AlGaInP LEDs during emission showed a very even dstribution of radiation covering the whole surface of the lode, in contrast to the standard LEDs in whch most of the radiation is emitted around the metal contact, resulting from current crowdmg effects. rhis shows the effectiveness of IT0 as a current spreadmg layer in LED structure. The light intensity versus both dc drive and pulsed current curves of ITO/AlGaInP are compared in Figure (5). During operation the heat generated at the active regon, under pulsed condtions of a few hundred nanoseconds causes a neghgible temperature rise. As the pulse width is increased, the intemal quantum efficiency decreases due to an increase in non-radiative recombination, and the overflow of injected carriers from the active region. These changes cause the light intensity to decrease and underlines the importance of the removal of heat generated at the active regon during device operation in order to obtain hgher radiative efficiencies. A slight deviations between the pulsed and the dc operation (0 - 0.02 A) observed, is as a result of junction temperature increase due to joule heating. The deviation observed on the standard LEDs are up to 20% (not shown), confirrmng the effect of lower joule heating effects in the ITO/AlGW LEDs. The electroluminescence spectral response of the diode measured at various forward currents (20, 50 and. 100 mA) is shown in Figure (6).The diode emits orange light, with a peak wavelength (hp)of 600 nm and full width at half maxi"of 15 nm respecttvely. There is a slight shift in the peak wavelength and increase in FWHM or the broadening of the peaks as the injection current is increased from 20 mA to 100 mA. Shifts in peak wavelength towards longer wavelength is usually attributed to band gap shrinkage caused by joule heating. These two effects were observed to be greater by more than 30% in the case of the standard LEDs. This hrther supports the hypothesis that LEDs with IT0 would benefit from a reduction in junction heating similar to In203 LEDs[5]. The peak wavelength (+-J is determined primarily by the band gap composition of the active layer.
CONCLUSION
In conclusion, the low sheet resistance, excellent transparency and refractive index matching of the IT0 enables it be used both as a suitable contact material in surface emitting LEDs, where uniform dstribution of current and the requirement of a window for maximum light extraction are crucial. A hrther advantage obtained is that of a lower forward resistance and junction heating compared to the standard LEDs, without the IT0 layers. Total light output in LEDs has been found to be dependent on the thickness of p-type AlGaInP cladding layer and total device area. The simplicity and ease of reactive thermal evaporation deposition techtuque at low temperature used in t h ~ swork has a number of advantages over the widely used sputtering techmques whch causes undesirable surface damage. Acknowledgements
We acknowledge the EPSRC and the Epitaxial Products International, St. Mellons, Cardiff U.K for the support of this work 84
REFERENCES
n+-GaAs Substrate
Figure 1: Schematic diagram of TTO/AlGalnP LED (n0tto.crk)
Contact
rype
TABLE (1) I Showing all combinations of Ca.& u
p layer and the n and p-type cladding layers tkknesses used in the device structures.
10'
'
I
I
1 oo
0
I
iTO/AICaInP LED
lo-' 1 o-2 1 o-3 1o
-~
1o
-~
1 o-6
IO-' lo-* 1o
- ~
1 o-'O
i-
8 8
T A
2
m
B
h
150
v
."h U
v)
F.
2
-c
100
aJ
.-
.U
Q
e, p:
50
0 1.65
1.70
1.75
1.8G
1.85 1 9J
1 95
2.C:
Voltage (V) Figure (4) Light-Voltage (L-V) characteristics for all t h e combination of cladding layer ana GaAs thicknesses
_7
%.-
,
:“f 200
1
I5O 100
0
1
i
A
0.00
I
0.02
.
l
.
,
.
I
0.06
0.04
.
0.08
,
,
0 . 12
0.10
Current (A)
Figure@): Light output intensity against dc current for ail cornhination of cladding laver and GaAs can laver thicknccscc
.
160
140 120 100
I
I
I
t
I
(300
t
Pulse
m
,./
1
I i
i
P’ P’
80
--/ 1 40
,
0
0.000
0.005
0.015
0.010
0.025
0.020
Current ( m A ) Figure(6) Pulsed and dc performance curves as function of light intensity
560
SSO
620
Wavelength (nm)
Figure 7: Spectral response at various forward currents of 20,50 and 100 mApf the ITOlAlGalnP LED
..-
64
