This invention relates to semiconductor devices such as light-emitting diodes (LEDs) and transistors, particularly to those employing nitrides or nitride-based compounds as semiconductors, and to a method of making such semiconductor devices.
Nitride-based semiconductor devices are usually built upon substrates of either sapphire, silicon carbide, or silicon. Silicon in particular offers the advantages of being less expensive and easier of dicing than sapphire or silicon carbide. Unlike sapphire, moreover, silicon can provide an electroconductive substrate that serves as part of the main current path through the device. Offsetting these advantages of the silicon substrate is a relatively great voltage drop caused by the potential barrier between the silicon substrate and the nitride semiconductor layers grown thereon. The nitride semiconductor LEDs with the silicon substrate have therefore required a drive voltage that is high enough to overcome the voltage drop.
Japanese Unexamined Patent Publication No. 2002-208729 teaches an LED configuration designed to preclude the noted shortcoming of the silicon substrate. It employs an n-type silicon substrate on which there is grown by epitaxy the main semiconductor region (i.e., region where light is generated) of the LED via a buffer region. The buffer region comprises an aluminum nitride (AlN) layer directly overlying the n-type silicon substrate, and an indium gallium nitride (InGaN) layer of the same conductivity type as that of the silicon substrate on the AlN layer. The main semiconductor region of the LED typically has an active layer of InGaN sandwiched between a lower cladding or confining layer of n-type gallium nitride (GaN) and an upper cladding or confining layer of p-type GaN.
In the course of the epitaxial growth of the successive layers of the buffer region and main semiconductor region of the above prior art LED on the n-type silicon substrate, there occur partial diffusions of aluminum from the lowermost AlN buffer layer, and of indium and gallium from the overlying InGaN buffer layer, into the silicon substrate. The result is the creation, at and adjacent the interface between silicon substrate and AlN layer, of a layer of the alloys or compounds of gallium, indium, aluminum and silicon. This alloy layer, as it might be so called, is in itself conducive to enhancement of LED efficiency as it reduces the potential barrier of the heterojunction between silicon substrate and AlN layer and so enables the LED to operate with a lower drive voltage than in the presence of the potential barrier discussed above. The LED is thus made less in power loss and higher in efficiency.
The trouble, however, is that aluminum, indium and gallium diffuse deeper down into the silicon substrate from the alloy layer. These Group III elements represent p-type impurities in the n-type silicon substrate, so that a pn junction was conventionally created in the substrate under the alloy layer. This applicant has ascertained that the pn junction caused a forward voltage drop of 0.6 volt or so. The residual potential barrier between the silicon substrate and the nitride semiconductor layers thereon was still so high that the voltage drop across this prior art LED (in other words, its drive voltage) was approximately 1.2 times as great as that across the sapphire-substrate LED.
This shortcoming of the n-type silicon substrate in conjunction with the nitride semiconductor layers grown thereon has manifested itself with LEDs of different make having no alloy layer between substrate and buffer region. The same problem has been encountered not only with LEDs but additionally with other types of semiconductor device such as transistors in which the current flows through the silicon substrate in its thickness direction.
Another problem with LEDs has been how to create an electrode on the light-emitting surface of the LED so as to fulfill the dual, or even self-contradictory, requirement of enabling the emission of light therethrough and providing electric connections with external circuitry. A typical conventional solution was to overlay the light-emitting surface with a transparent sheet or film of a material such as a mixture of indium oxide and tin oxide and to place an opaque, metal-made wire-bonding pad centrally on the transparent overlay. The metal of the bonding pad is easy to diffuse into the transparent overlay and even into the underlying main semiconductor region, the transparent overlay being as thin as, say, ten nanometers. A Schottky barrier was therefore created between the bonding pad and the main semiconductor region. Capable of blocking the forward current of the LED, the Schottky barrier reduced the amount of current flowing right under the bonding pad and added to the amount of current flowing through the outer part of the semiconductor region which is out of register with the bonding pad.
Thus the Schottky barrier functions just like the current-blocking layer which has been conventionally formed under the bonding pad for preventing current flow there. The current flowing right under the bonding pad is a waste of energy because the light generated there is blocked by the opaque bonding pad. Only the light produced at the outer part of the main semiconductor region issues from the LED through the transparent overlay. The greater the proportion of the current flowing through the outer part of the main semiconductor region, the higher will be the efficiency of the LED.
The n-type silicon substrate necessitates as aforesaid the application of a high drive voltage forwardly of the LED, which is tantamount to more power losses and more heat production at both silicon substrate and main semiconductor region. The noted Schottkey barrier deteriorates as a result of the heating, permitting greater current flow therethrough and, in turn, causing less current flow through the outer part of the main semiconductor region. With the reduction of current flow through the outer part of the semiconductor region, a correspondingly less amount of light is generated and emitted through the transparent part of the electrode. The prior art LEDs having nitride semiconductors on n-type silicon substrates were therefore mostly unsatisfactory in the efficiency of light emission.
It has been known and practiced to create a dedicated current blocking layer of electrically insulating material between the bonding pad and the main semiconductor region. This solution is objectionable on account of the additional manufacturing steps, and the consequent higher manufacturing costs, needed for fabrication of the current blocking layer.
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