American corporations are becoming increasingly interested in investing in Africa, with some regarding it as the last big growth market, but they are often deterred by Africa’s negative image, a new study shows.  US companies in some sectors, particularly technology companies, now regard Africa as "the last frontier for growth."   It has a market of one billion people, mobile telephone networks have been successful, and other countries, particularly China, are increasing their African investment thrust.   Africa is the second largest and second most populous continent after Asia, with 20% of the world’s land area and 14% of global population.